Atomic Absorption Spectrometer - Globallab Book.pdf · Atomic absorption spectroscopy (AAS) relies on the fact that the light ... standards allows a calibration curve to be constructed

COOKBOOK – Book One

AI 1200Atomic Absorption Spectrometer

Updated: Jan 2002

AI 1200 Cookbook Table of Contents

AI 1200 COOKBOOK

Table of Contents

BOOK ONE- FAAS Chapter 1: Theory of AAS

Introduction 2 Flame Atomic Absorption Spectrometry (FAAS) 3 Graphite Furnace Atomic Absorption Spectrometry (GFAAS) 4 Vapor Hydride Generation Atomic Absorption Spectrometry (VG AAS) 5

Chapter 2: AAS Instrumentation

Fundamentals 9 Light Source Atomizer Optics Detector

Optics 10 Lenses Mirrors Monochromator Diffraction Grating Slit Width

Atomizer 15 Flame Graphite Furnace

Detector 18 Chapter 3: Background Correction

Fundamentals 22 The Frequency of Measurement The Interval between Measurements The Function used to Calculate Net Absorption Spectral and Structured Backgrounds The Effect on the Linear Working Range Deuterium (D2) Background Correction 24 Smith-Hieftje (S-H) Background Correction 27 Zeeman Background Correction 28 Comparison of Background Correction Methods 29

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Chapter 4: Comparison of Analytical Techniques

Things to Consider 34 Applications 34 Expected Concentration Ranges 34 Elements 34 Atomization Efficiency 34 Interferences 35 Spectral Background Matrix Detection Limits 35 Sensitivity 35 Precision 35 Linear Working Range 35 Minimum Sample Volume 36 Sample Throughput 36 Sample Usage 36 Total Dissolved Solids 36 Method Development 36 Ease of Use 36 Automation/Unattended Operation 36 Costs 37 Initial Investment Running Costs

Chapter 5: Standard and Sample Preparation

Apparatus 41 Water 41 Standard and Blank Solutions 41 Sample Solutions 42 Storage of Solutions 42 Calibrations 43 Matrix Effects 43 Chemical Interferences 43

Incomplete dissociation of analyte compounds Ionization

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Chapter 6: FAAS Analytical Data Sheets

Introduction 47 THE ELEMENTS

Aluminum, Al 49 Antimony, Sb 50 Arsenic, As 51 Barium, Ba 52 Beryllium, Be 53 Bismuth, Bi 54 Boron, B 55 Cadmium, Cd 56 Calcium, Ca (air/acetylene) 57 Calcium, Ca (nitrous oxide/acetylene) 58 Cesium, Cs 59 Chromium, Cr (air/acetylene) 60 Chromium, Cr (nitrous oxide/acetylene) 61 Cobalt, Co 62 Copper, Cu 63 Dysprosium, Dy 64 Erbium, Er 65 Europium, Eu 66 Gadolinium, Gd 67 Gallium, Ga 68 Germanium, Ge 69 Gold, Au 70 Hafnium, Hf 71 Holmium, Ho 72 Indium, In 73 Iridium, Ir 74 Iron, Fe 75 Lanthanum, La 76 Lead, Pb 77 Lithium, Li 78 Lutetium, Lu 79 Magnesium, Mg 80 Manganese, Mn 81 Mercury, Hg 82 Molybdenum, Mo 83 Neodymium, Nd 84 Nickel, Ni 85 Niobium, Nb 86 Osmium, Os 87 Palladium, Pd 88 Phosphorous, P 89 Platinum, Pt 90 Potassium, K 91 Praseodymium, Pr 92 Rhenium, Re 93 Rhodium, Rh 94 Rubidium, Rb 95 Ruthenium, Ru 96 Samarium, Sm 97

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Scandium, Sc 98 Selenium, Se 99 Silicon, Si 100 Silver, Ag 101 Sodium, Na 102 Strontium, Sr 103 Tantalum, Ta 104 Tellurium, Te 105 Thallium, Tl 106 Tin, Sn 107 Titanium, Ti 108 Tungsten, W 109 Uranium, U 110 Vanadium, V 111 Ytterbium, Yb 112 Yttrium, Y 113 Zinc, Zn 114 Zirconium, Zr 115

Chapter 7: Practical Applications - FAAS

Marine 120 Water 121 Biological 122 Food 123 Agricultural 124 Petroleum 127 Miscellaneous 129

References 132

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AI 1200 Cookbook - Book One 1 - Theory of AAS

Chapter 1: Theory of AAS

• Introduction • Flame Atomic Absorption

Spectrometry (FAAS) • Graphite Furnace Atomic Absorption

Spectrometry (GFAAS) • Vapor Hydride Generation Atomic

Absorption Spectrometry (VG AAS)

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Introduction

The essential elements of the theory behind the analytical technique of atomic absorption spectroscopy (AAS) are compacted into the following paragraphs.

Atomic absorption spectroscopy (AAS) relies on the fact that the light

absorption of free atoms [1-7]. All atoms can absorb light, but only at discrete wavelengths corresponding to the energy requirements of the particular atom. In other words, each element absorbs light at some specific and unique wavelength and does not absorb light at all on other wavelengths. For example, in a sample with multiple elements (say, copper, lead, iron, and nickel), only copper will absorb light that is at the characteristic wavelength for copper. Furthermore, the amount of light absorbed depends on the number of absorbing atoms that are present in the light path. All these factors enable AAS to be used as a tool for quantitative analysis.

In practice, measuring the amount of light absorbed by several known

standards allows a calibration curve to be constructed. Then, the unknown concentration of a sample can easily be determined based on the amount of light it absorbs.

The amount of light energy absorbed at this wavelength depends on the concentration of the atoms in the medium (as dictated by Lambert’s law and Beer’s law). Lambert’s law states that the portion of light absorbed by a transparent medium is independent of the intensity of the incident light and each successive unit layer of the medium absorbs an equal fraction of the light passing through it. Beer’s law states that the light absorbed is proportional to the number of absorbing atoms in the medium. Mathematically, when light of intensity Io passes through a medium of length x with atom concentration of C, the intensity I of the light beam emerging from the medium is given by:

I = Io e-kCx

where k is a proportionality constant (the absorption coefficient). The absorption of the medium, A, is defined to be:

A = lg (Io/I) = kCx

This equation states that the absorbance, A, of the medium is linearly

proportional to the concentration of the absorbing atoms. The absorption coefficient (or absorptivity), k, can be determined by constructing a calibration curve (i.e. plotting the observed absorbance versus the known sample concentration). The slope of the calibration curve is kx, and x is easily measurable or already known. Unknown sample concentrations may be determined from the calibration curve based on their measured absorbances. Any way that you look at, every AAS experiment can be broken down to the following procedure:

• A sample has an unknown amount of a known element (e.g. the sample is known to contain lead, but not how much lead). The sample must be made into a homogeneous, liquid solution (if it is not already).

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• A blank solution must be prepared. This blank must contain none of the element of interest.

• A series of standard solutions must be prepared. These standards have known (but varied) concentrations of the element of interest. These standards are used to prepare a calibration curve.

• Analyze the blank solution to determine the "blank" absorbance value. This is the absorbance value for a sample with a zero concentration of the element of interest.

• Individually analyze all of the standard solutions. • Construct a calibration graph. For the blank and each standard solution, plot

its absorbance value against its concentration. • Analyze the unknown sample. The concentration of the unknown sample,

based on its measured absorbance value, can be determined from the calibration curve.

There are three main techniques of atomic absorption spectrometry: Flame

atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry (GFAAS), and vapor hydride generation atomic absorption spectrometry (VG AAS). Each has its own distinct advantages and disadvantages. Each has specific applications for which they are the superior AAS technique. For example, FAAS is suitable for analyses where the sample is above trace quantities, and where high sample throughput, ease of use, and low initial investment are required. GFAAS is ideal for samples that are in the parts per billion (ppb) range and where the sample volume is limited. VG AAS is useful for determining elements that form volatile hydrides at sub-trace levels. A brief outline of each technique is provided below. Flame Atomic Absorption Spectrometry (FAAS)

The atomization process by which the atom population is generated is of

primary importance in AAS because analysis depends entirely on the fact that free, uncombined atoms will absorb light of a particular wavelength. The key to successful operation of an atomic absorption spectrometer lies in generating a supply of free, uncombined atoms in the ground state and exposing this atom population to light at the characteristic absorption wavelength. The source of energy for free atom production is heat, most commonly in the form of an air-acetylene or nitrous oxide-acetylene flame (Flame AAS). With this type of atomizer, the sample solution is introduced in the form of a spray of fine droplets. This is accomplished by a pneumatic nebulizer in most case. The spray of droplets is carried by a gas (usually the oxidant for the flame) through the spray chamber and burner head into the flame. The heat of the flame is sufficient to dry (desolvate) each of the sample droplets and (usually) to decompose chemical components from the resulting dried particles into their constituent atoms. Thus a population of ground state atoms is created in the flame and atomic absorption measurements can be made.

Flame systems for AAS give excellent results, and they are simple, inexpensive, convenient and extremely useful. They permit rapid analytical measurements through a very simple sample introduction technique. The major limitation of flame AAS is that the burner-nebulizer system is a relatively inefficient sampling device. Only a small fraction of the sample that is taken up reaches the flame. Additionally, once atomized, the sample passes quickly through the light path. An improved sampling device would atomize the entire sample and retain it in the light path for an extended period of time to enhance the sensitivity of the

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technique. Electrothermal atomization using a graphite furnace provides these features. Graphite Furnace Atomic Absorption Spectrometry (GFAAS)

Graphite Furnace Atomic Absorption Spectrometry (GFAAS) has gained a

reputation in the field of analytical chemistry as a routine technique for the determination of very low levels of trace metals in a variety of sample matrices. With GFAAS, the flame has been replaced by an electrothermally heated graphite tube. The sample is injected directly into the tube as a small liquid volume (5 to 100 µL), which is then heated in a programmed series of steps to remove the solvent and major matrix components. Free analyte atoms in the gaseous state are eventually produced inside the graphite furnace by rapidly heating with a strong electric current to temperatures between 1500 and 3000°C. All of the analyte is atomized, and the atoms are retained within the tube (and the light path, which passes through the tube) for an extended period (typically 0.2 to 0.5 second). The performance of this technique relied on the stability of the temperature. The recently developed transversely heated integrated contact graphite furnace ensures the temperature over the entire length of the tube is very uniform. As a result, sensitivity and detection limits are significantly improved while matrix interferences and memory effects are reduced. After the measurement, the analyte vapor is then purged from the graphite furnace by helium or argon gas. The magnitude of the absorbance is recorded as a function of time by the readout system.

The mechanism of atomization in a graphite furnace depends significantly on the chemical nature of the analyte element, the availability of active sites, the gaseous species within the graphite furnace, the atomization temperature, and the graphite furnace itself. After drying and pyrolysis the analyte atoms may be present in reduced, oxidized or complex form. Upon heating the graphite tube to the atomization temperature, free analyte atoms are generated by:

(1) vaporization of the reduced form from the surface, (2) by dissociation of the oxide form into gaseous free analyte atoms as the

graphite tube heats up, and (3) vaporization as oxides (or any other molecular species) and dissociation

into free analyte atoms in the gaseous phase.

Examples of these mechanisms are given below - where M is an analyte, C is carbon and O is oxygen and subscripts g and s refer to gaseous and solid phases:

M (s) ➨ M (g)

MO (s) + C(s) ➨ M (g) + CO (g) MO (s) ➨ MO (g) MO (g) ➨ M (g) + O (g)

CO (g) + O (g) ➨ CO2 (g).

Analysis times for GFAAS are longer than those for FAAS, and fewer elements can be determined with this technique. Nonetheless, GFAAS’s enhanced sensitivity, ability to analyze very small sample sizes, and ability to directly analyze certain types of solid samples significantly expand the capability of atomic absorption spectrometry.

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Vapor Hydride Generation Atomic Absorption Spectrometry (VG AAS)

For the determination of As, Bi, Ge, Pb, Sb, Se, Sn, Te and Hg with AAS,

vapor/hydride generation (VG) techniques have been proven to provide very high sensitivities and reduced interferences. With VG AAS, analytes are first reduced to their corresponding volatile hydrides (or metallic form for Hg) by sodium borohydride in an acidic medium. The vapors are then transported by a carrier gas into the atomizer for atomization and AA measurement.

Aurora Instrument’s AI 1200 uses an open ended, temperature controlled electrothermally heated quart tube for continuous flow vapor/hydride generation determinations. The heating unit can be installed and removed easily.

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AI 1200 Cookbook - Book One 2 - Instrumentation

Chapter 2: AAS Instrumentation

• Fundamentals • Light Source • Atomizer • Optics • Detector

• Optics • Lenses • Mirrors • Monochromator • Diffraction Grating • Slit Width

• Atomizer • Flame • Graphite Furnace

• Detector

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Fundamentals There are four components that are essential to every AAS instrument: a light source, an atomizer, an optics system, and a detector. Light Source Usually a hollow cathode lamp (HCL) is used as the light source in AAS. A less common light source is the electrodeless discharge lamp (EDL). An HCL produces an intense, narrow line emission of light at a wavelength that is specific to the element that the HCL cathode is coated with. Most elements emit at multiple wavelengths, but all emissions are intense, sharp lines (called resonance lines). For example, a copper HCL emits at 324.75 nm, 327.40 nm, 222.6 nm, 249.2 nm, and 244.2 nm. Generally, one line is more intense than the others and is therefore the most sensitive and useful line for AAS analysis. HCLs are coated with the element of interest to produce light of the resonance wavelength(s) that is/are specific to element. When this light is passed through a medium that contains atoms of the same element, the light will be partially absorbed. Atomizer The purpose of an atomizer is to create a population of free atoms that is suitable for absorption of light. The atomizer must have an energy source in order to do this. Usually the energy comes from heat, and the most common source of the heat is a flame (either an air/acetylene or a nitrous oxide/acetylene flame). An AAS instrument with a flame atomizer is called a flame atomic absorption spectrometer (FAAS). With a flame atomizer, the sample is introduced into the flame as an aerosol (a mist of tiny droplets). The flame burner head [8] is designed to be long, thin, and aligned with the light path. Such a design causes the aerosol atoms to be atomized in the flame while they are in the light path so that they can absorb the light. A very important part of the atomizer system is the nebulizer. The nebulizer is responsible for nebulizing a liquid sample into an aerosol. The sensitivity of a FAAS instrument depends heavily on how efficiently the nebulizer can convert a sample to an aerosol. Optics The optics system of an AAS instrument is responsible for getting the light from the light source to the detector. Along the way, the light must be passed through the atomized sample and through a monochromator. A monochromator is used to isolate specific wavelengths from the bulk light that it receives. For example, it may be necessary to isolate the analytical wavelength of interest from light that was emitted from the fill gas of the HCL, or from stray room light that entered the spectrometer. Detector The detector part of an AAS instrument measures how much light is transmitted through the spectrometer. Most commonly a photomultiplier tube (PMT) is employed for this purpose. While the above four components are the essential ones of an AAS instrument, there are still others that play important roles.

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There inevitably are electronic devices that convert the signals from the detector into something that is useful for the human researcher. Older instruments used to employ signal meters and plotters that would chart the strength of the absorbance signal on a moving strip of paper. Instruments of today have replaced the meter and chart recorder with computer software that has many more capabilities. Modern software provides real-time plots of absorbance versus time, constructs calibration curves, and calculates statistics such as RSD values. On the starting end of the sample analysis spectrum, computer software can keep track of samples that are running, that you will run, and that you did run. It can also be used to setup and run the instrument without operator intervention at all. Optics

The ideal optics system will have the following characteristics: • Have 100% efficient light throughput. In other words, if there is no

atomized sample in the light path, then 100% of the light from the source will reach the detector.

• Allow zero stray light. • Provide a high signal to noise ratio (S/N). • Provide absolute selectivity and resolution for the wavelength being

measured. • Provide constant dispersion, regardless of wavelength. • Have no optical aberrations. • Provide unique performance over a wide wavelength range.

Unfortunately, there is no such thing as an ideal optics system. The best that

you can hope for is to have a system that is optimized for your needs. There are two ways to control the path of light within an optics system: with

mirrors or with lenses. Most optics systems make use of both.

Lenses Good quality lenses are made from silica glass and have good light

transmission over a broad wavelength range (~190 - 900 nm). Transmission losses occur at both interfaces of the lenses (i.e. at the lens surface where the light enters and at the lens surface where the light exits). The losses are typically between 10-14% for each lens in the optical path.

A feature of lenses that must be kept in mind is that the refractive index of the lens is dependent on the wavelength of the light being refracted. This means that the focal length of the lens will be different for every wavelength. Rather than moving a lens to keep the focal point in the same position for different wavelengths, optics systems will keep their lenses fixed and tolerate the relatively minor losses associated with the changing focal lengths.

Most lenses used are designed for wavelengths in the UV region. This is because most analytical wavelengths are in this range and the median refractive index is about 250 nm.

For an air/acetylene flame, where the path length is around 10 cm, the losses due to focal length differences are negligible. For a nitrous oxide/acetylene flame, however, the path length is only 5 cm and the losses can start to become noticeable. In GF AAS, where the sample atomization occurs

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only at a small point in the center of the graphite tube, the focal length from the light source is absolutely critical. As the light source wavelength is increased, the focal point will move away from its original position at the center of the graphite tube. Because of this, optics systems for GF AAS systems usually employ only mirrors, and not lenses.

Mirrors Mirrors perform much better than lenses, both in terms of reflecting

efficiency and focal length change. A mirror has a very thin (e.g. 2 µm) top coating of aluminum that reflects more than 90% of the light that strikes it (for the range 190 - 900 nm). Also, when light reflects off a mirror there is no change in the focal length. That is, the focal length of a mirror depends only on its shape and is independent of wavelength.

Plane mirrors are used to fold light and curved mirrors (also called collimating mirrors) are used to focus light. For example, a plane mirror is needed to fold light around a 90° corner, and a curved mirror is needed to focus that light onto an entrance slit. While the use of mirrors does solve the problem of focal length differences, it raises the challenge of designing and manufacturing focusing mirrors that are free from other optical aberrations, such as astigmatism.

Because of the thinness of the mirror coating, mirrors are extremely fragile and must be handled with care. Finger prints, and even soft tissues, can irreversibly damage a mirror’s coating. Reactive liquids and gases can even cause harm. To increase their longevity, most optics mirrors are further coated with a transparent silica or magnesium fluoride film for protection. Even still, one should avoid any kind of direct contact with mirror surfaces.

Monochromator

There are several different designs of monochromators available. No matter which design is used in an AAS instrument, however, some fundamental principles remain the same. Light enters the monochromator through an entrance slit. The light is folded and focused in the monochromator by use of mirrors. The light is dispersed into its component wavelengths by some sort of diffracting element. The diffracted light then leaves the monochromator through an exit slit.

The most common monochromator design is the Czerny-Turner design,

which is used in the AI 1200. A schematic of this type of monochromator is shown in Figure 2.1.

The Czerny-Turner monochromator uses two separate mirrors to collimate and focus light. Mirror #1 receives the light that was focused through the entrance slit. The light that reflects off this mirror is collimated into parallel beams and then strikes the diffraction grating, which diffracts the light into a spectrum of wavelengths. This spectrum is dispersed at a variety of angles, depending on the wavelength of each component of the spectrum. The light then strikes Mirror #2, which focuses the light through the exit slit and into the detector. Some monochromators make use of only one mirror, but there is a definite advantage to using two mirrors. The two mirrors will each be smaller than a single mirror, so they are easier to manufacture. This means that there is less chance for surface aberrations and therefore allows optimum light throughput and resolution.

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Figure 2-1 Schematic diagram of a Czerny-Turner monochromator

The detector is dumb — it doesn’t know whether the light it receives is an

analytical signal or not. The detector merely counts the number of photons that it receives (the intensity of the light) and sends a signal to an amplifier then sends back to computer. So, it is the job of the monochromator to isolate a specific, narrow resonance line from the rest of the spectrum before the light is allowed to reach the detector. The monochromator ensures that only the analytical wavelength of light reaches the detector. This wavelength selectivity is achieved by rotation of the diffraction grating with respect to the incident light. Turning the grating moves the spectrum across the exit slit, and therefore changes the wavelength of light that passes through the slit. Another means of controlling the light that passes through the exit slit is by changing the width of the slit. A narrower slit allows two closely spaced wavelengths to be resolved, but it also decreases the light throughput of the optics system as a whole. If there are no interfering wavelengths close to the analytical wavelength, then a wider slit can be used to increase the light throughput, say, improve the signal noise ratio. Other, less common used monochromator designs are the Ebert-Fastie and the Littrow designs. The Ebert-Fastie monochromator design makes use of a single, large mirror to focus light. See Figure 2.2 for a schematic. One area of the mirror collimates incoming light onto the diffraction grating and then another area of the mirror focuses the light dispersed from the grating onto the exit slit. This single mirror does both of the jobs of the double mirrors in the Czerny-Turner design. A disadvantage of the Ebert-Fastie design is that the single mirror must be large. This increases the probability of surface imperfections during the manufacture of the mirror, which in turn impairs the light throughput and resolution of the monochromator. An Ebert-Fastie monochromator is, however, less expensive than a Czerny-Turner.

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Figure 2-2 Schematic diagram of an Ebert-Fastie monochromator

The Littrow monochromator design is similar to the Ebert-Fastie design in that it also employs only one mirror. See Figure 2.3 for a schematic. The difference lies in that the Littrow monochromator uses the same area of the single mirror for collimating the light onto the grating as for focusing the dispersed light onto the exit slit. This fact increases the chances of optical aberrations (even more so than the Ebert-Fastie design).

Figure 2-3 Schematic diagram of a Littrow monochromator Diffraction Grating

There are two types of diffraction gratings that are commonly used today in AAS instruments: ruled gratings and holographic gratings. Ruled gratings have been around longer than holographic gratings, which were only introduced in the late 1960s. A grating, in general, is a closely spaced series of grooves in a flat, reflecting surface. The grooves must be perfectly parallel and uniformly

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spaced from each other. The closer the grooves are spaced, the better the resolving capability of the grating. Gratings are produced with groove densities from 500 to 6000 grooves/mm. The blaze wavelength of a diffraction grating is the wavelength of light that will be most efficiently diffracted by the grating. Generally, gratings can be used to diffract light that is 2/3 to 3/2 of the blaze wavelength. For example, consider a grating blazed at 400 nm. The grating can be used for wavelengths from 270 nm to 600 nm, but will diffract most efficiently wavelengths of 400 nm.

Ruled grating are physically etched, groove-by-groove, by a machine. Basically, a mirror is mounted on a grooving machine, a diamond bit etches a straight groove, the mirror is moved a short distance and then the bit etches another straight groove parallel to the previous one.

Holographic gratings, on the other hand, are manufactured with light, not a physical machine. A piece of glass is coated with a light-sensitive material, which is then exposed to two parallel beams of coherent light that produce an interference pattern on the coated glass. The bright areas of the interference pattern (where the light beams add constructively) form the “grooves” in the developed photoresist. A thin layer of aluminum is then applied onto the “etched“ glass piece to form a mirrored, grooved surface — a diffraction grating. The advantage of the holographic technique over the machine etching technique is that the holographic technique produces no systematic errors, since the grooves are the result of a perfect optical phenomenon. The machined technique, on the other hand, is only as good as the quality of the etching machine itself (which, for many purposes, is excellent, but will never be truly “perfect”).

Gratings that are used in monochromators are always copies of “master” gratings. A master grating is the original grating that was manufactured (either holographically or physically). Subsequent gratings can be made from the master by a process that essentially makes a molded copy of the master.

There are advantages and disadvantages to both types of gratings. The appropriate one to use in a monochromator depends on several factors. Ruled gratings produce significant more stray light than holographic gratings, and this is especially true when groove density increases. For this reason, the maximum groove density of rules gratings is around 3600 grooves/mm. Holographic gratings can have up to 6000 grooves/mm. So, based on this factor, a holographic grating is better to use if a higher groove density is required to achieve a higher resolution and maintain a high signal to noise ratio.

Ruled gratings do exhibit significantly better efficiency than holographic gratings. So, based on this factor, a ruled grating is more appropriate to use if light throughput is critical (for example, if the light source is very weak). If the light source is intense (as is the case in AAS instruments), then using a holographic grating is more advantageous than using a ruled grating.

Slit Width The slit width affects how much light enters and exits the monochromator,

and so is very important for light throughput. A wide slit width will allow more light to reach the detector and will improve the signal strength. But, if non-analytical lines also reach the detector, then this will increase the noise and decrease the signal to noise ratio. Conversely, a narrow slit width will block out all non-analytical wavelengths, but may reduce the light throughput so much to make the signal to noise ratio unsatisfactory. So, the best slit width is the one that allows the most light to reach the detector and blocks out most of the noise.

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In other words, finding the optimum slit width is a compromise between maintaining high light throughput and maintaining a high signal to noise ratio.

The maximum allowable slit width is generally determined by how closely spaced the analytical line of interest and its nearest neighbor in the spectrum are. The slit width must be narrow enough to block out any non-analytical lines, since they would increase the noise that the detector would see. Every spectrum is different, so the optimum slit width must be determined for each analysis. For elements whose analytical wavelengths are high (for example, rubidium at 780 nm), the spectra are usually not as dense as for elements with shorter analytical wavelengths (for example, zinc at 214 nm). Therefore, a larger slit width is usually permissible for elements like rubidium, whereas elements like zinc usually necessitate narrower slit widths. Atomizer

The atomizer is arguably the most important part of an AAS instrument, since this is where the sample is converted into atoms that can absorb light. Having an efficient atomizer is essential. The absorbance signal is completely dependent on how many atoms there are to absorb the source light. So, a good atomizer will display good sensitivity to the sample being analyzed, and a poor atomizer will display poor sensitivity.

The ideal atomizer has the following characteristics: • Atomizes 100% of the sample delivered to it. • No ionization occurs. • No sample is left in the molecular, complexed form. Of course, no real atomizer is ideal, and the degree to which any sample

is atomized depends to a large extent on the element being analyzed. Atomization is achieved by heating the sample to an extent where free ground state atoms are formed. In FAAS, atomization is done with a flame. In GFAAS, atomization is done with an electrically heated graphite tube furnace. In VG-AAS, atomization is done with an electrothermally heated furnace or a flame.

Flame The atomizer in a FAAS instrument uses a nebulizer to convert a liquid sample into an aerosol. The sample is introduced into the nebulizer by aspiration though capillary tubing. The aspiration occurs pneumatically from the flow of fuel and oxidant gases through the nebulizer chamber. After the sample has traveled through the capillary tubing, it strikes a glass impact bead. This impact bead is designed so that when a stream of liquid strikes it, the liquid breaks apart into a mist of drops (an aerosol). This aerosol invariably contains drops of many sizes. The larger drops fall out, but the smaller drops remain suspended and are thoroughly mixed with the fuel and oxidant gases as the mixture is carried into the spray chamber. As the sample mist gets mixed with the gases, it moves along through the spray chamber and up towards the burner head. The sample entering the burner head is a uniform mixture of fuel gas, oxidant gas, and tiny sample droplets. Once the mixture enters the flame, the process of atomization by heat begins. The heat from the flame is usually sufficient to desolvate the sample droplets. Then, the solid particles that were formed (e.g. salts) are broken down, melted, or volatilized into gases. Finally, the molecules are thermally dissociated into atoms that are capable of absorbing their characteristic wavelength of light. Of course, the heat from the flame may be excessive and

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may ionize some atoms (remove an outer electron from the atom), thus decreasing the absorption signal. As well, the heat from the flame may be insufficient and not atomize enough of the sample molecules. This also decreases the absorption signal. The process of thermally atomizing the sample in the flame is a complex equilibrium, and the actual chemistry inside the flame at any stage (especially the atomization stage) is not clear. There are often numerous side reactions that occur simultaneously. The degree to which a sample may be ionized in a flame depends on the element, since each element has different energy requirements for ionization. To reduce ionization of atoms, easily ionizable elements (EIEs), such as the Group I elements (Li, Na, K, Cs) can be added to the original sample solution. If the EIEs are much more easily ionized than the sample atoms, then they will create a large population of electrons in the flame and shift the atomization/ionization equilibrium of the sample atoms towards the atomization side. There are two types of flames that are commonly used for FAAS: air/acetylene and nitrous oxide/acetylene. The air/acetylene flame (air being the oxidant, acetylene being the reductant, or fuel) burns at around 2300 °C. The nitrous oxide/acetylene flame burns much hotter at around 3000 °C. So, the flame temperature is a factor when determining which type of flame is best suited for atomization of a given element. Because of its cooler burning temperature, the air/acetylene flame works well for elements that are relatively easily atomized, such as copper, iron, nickel, and gold. The nitrous oxide/acetylene flame, with its higher burning temperature, is needed to atomize elements that require more energy to atomize, such as aluminum, silicon, titanium, and tungsten. Another important factor in optimizing the atomization of an element in a flame is the stoichiometry of the oxidant and reductant gases. A lean flame is fuel poor, and is therefore an oxidizing flame. A rich flame has excess fuel, and is therefore a reducing flame. For each type of flame, certain elements are atomized best in reducing flames, and certain elements are atomized best in oxidizing flames. There is extensive data on the absorbance characteristics of all the elements in flames. For example, in a reducing flame there are excess carbon and hydrogen atoms present in the flame (from the acetylene molecules). These excess atoms help break down the strong oxide bonds that form with some elements, such as chromium. Other elements that are best atomized in a reducing flame are tin and molybdenum. On the other end of the spectrum, elements like silver, cadmium, gold, and nickel are best atomized in an oxidizing flame. Some elements, like iron and gallium, are best atomized in a stoichiometric flame (i.e. neither rich nor lean). Furthermore, some elements are satisfactorily atomized over a wide range of flame gas mixtures. Copper, for example, is atomized in both rich and lean flames. For this reason, copper is often used to test or validate the sensitivity of an AAS instrument. The major disadvantage of the flame atomization technique is that it is very inefficient at converting the original sample to atoms. Overall, the atomizer system of a FAAS instrument can convert less than 0.1% of the original sample to absorbing atoms. The nebulizer component usually transports only less than 10% of the aspirated sample into the aerosol, and the other 90% is lost as waste in the spray chamber and nebulizer chamber. Furthermore, the sample that does make it into the flame (in aerosol form) is already greatly diluted from its mixing with the flame gases. And once the sample gets atomized in the flame, the atoms residence times in the light path are extremely short. Atoms travel through the light path at great speeds (at least 1 cm/10 ms) as they exit the slit in the burner head and travel up the flame.

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Graphite Furnace

The graphite furnace (GF) atomizer solves the two major problems of the flame atomizer: poor atomization efficiency and short residence times. A graphite furnace is a tube that is connected to two low voltage electrodes. When a current is forced through the tube, the tube heats up. The amount of heating caused by the current flow can be accurately controlled, and the atomization of samples in a graphite furnace is usually performed over several heating steps. The graphite furnace is mounted in the electrodes so that one open end of the tube faces the light source and the other open end faces the entrance to the optics. This allows the light to pass freely through the graphite furnace along the axis of the tube. The tube is aligned so that the light path travels down the graphite tube axis and right through the center of the tube. With a GF atomizer, a very small amount of liquid sample (between 5 and 100 µL) is placed inside the center of the graphite tube. A heating program to atomize the sample consists of at least three principal steps:

1. Drying Step The graphite tube is quickly heated to a temperature just below the

boiling point of the solvent. Then the temperature is slowly ramped past the boiling point. This step gently evaporates the solvent (without causing splattering or sample ejection) and leaves the dried sample inside the tube. 2. Ashing Step (or Charring)

This step removes any dry or semi-dry matrix that is left over from the drying step. Matrix modifiers can be added to the sample before the heating program to stabilize the sample during the ashing step. Modifier gases, such as oxygen or hydrogen, can be added to the graphite furnace workhead during the ashing step to help remove the matrix. Ashing temperatures depend on the element being analyzed in its matrix. For example, cadmium ashes at 300 °C, arsenic at 1400 °C, iron at 600 °C, and lead at 480 °C. 3. Atomization Step At this stage, the sample is a dry solid at the bottom of the graphite tube. The sample is atomized by rapidly increasing the temperature of the tube. The AI 1200 can heat at a rate upto 3800 K/s. The required atomization temperature depends on the element being analyzed. For example, cadmium atomizes at 1250 °C, arsenic at 2250 °C, nickel at 2250 °C, and lead at 1400 °C.

There are distinct advantages to the graphite furnace atomizer when its performance is compared to the flame atomizer. The graphite furnace atomizes 100% of the sample (compared to less than 0.1% for the flame). Also, the residence time of the atomized sample in the light path is much longer in the graphite furnace than in the flame. The residence time in the graphite tube can range from 0.2 to 0.5 second, whereas in the flame it is only milliseconds. Both of these factors increase the sensitivity of the graphite furnace atomizer, and detection limits with GFAAS are typically one to two orders of magnitude better than with FAAS.

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Detector The detector is the part of the AAS instrument that receives the light output from the monochromator. The detector quantifies how much light it receives and creates an electrical current. That current is then amplified and converted into a digital signal that is recorded by a data acquisition system. By far the most common detector used in AAS instruments is the photomultiplier tube (PMT). Essentially, a PMT is a photon counter. Light from the monochromator enters the PMT through a quartz window. Photons (the quantum packets that make up light) strike the photocathode of the PMT. This converts the photon to a photoelectron via the photoelectric effect. However, the production of a single electron from a single photon won’t generate a very strong signal, so an amplification 5 or 6 orders of magnitude is required. This amplification is achieved through the use of a series of 8 to 12 dynode plates. A voltage is applied between the photocathode and first dynode (on the order of 100 V). This voltage difference causes the photoelectron to accelerate from the photocathode to the dynode. When it strikes the dynode, several more electrons are produced. These electrons are in turn accelerated towards the second dynode, since there is also a voltage difference across the first and second dynodes. Each of the electrons striking the second dynode creates several more electrons. For example, if the electron collision into the first dynode created 5 electrons, then the collision of those 5 electrons into the second dynode will create 25 electrons. This chain reaction of electron production continues along the series of dynodes. If there are 12 dynodes in the chain, then the original single photon will produce 244,140,625 electrons from the final twelfth dynode. These final electrons are then collected by the PMT anode. The current that can result from the collection due to a single photon by the PMT can be as high as 100 mA! Clearly, PMTs are extremely sensitive detectors. Furthermore, the large amplification of the signal that is achieved by the PMT is achieved with very little increase in noise.

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AI 1200 Cookbook - Book One 3 - Background Correction Methods

Chapter 3: Background Correction Methods

• Fundamentals • The Frequency of Measurement • The Interval between

Measurements • The Function used to Calculate

Net Absorption • Spectral and Structure

Backgrounds • The Effect on Linear Working

Range • Deuterium (D2) Background

Correction • Smith-Hieftje (S-H) Background

Correction • Zeeman Background Correction • Comparison of Background

Correction Methods

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AI 1200 Cookbook - Book One 3 - Background Correction Methods Fundamentals

Background correction is a necessary part of any good AAS instrument. Basically, there are two types of backgrounds that need to be corrected for: non-specific radiation and non-specific absorption.

Non-specific radiation was dealt with in Chapter 2. Non-specific radiation is the extra light (non-analytical wavelengths) that can pass through the optics of an AAS instrument, enter the monochromator, and have the chance of reaching the detector. If this non-specific radiation reaches the detector, it will result in a falsely signal. Non-specific radiation can come from many sources, including the HCL fill gas, sunlight, room light, and the light emitted by the flame itself. As was discussed in Chapter 2 (Monochromator section), it is the job of the monochromator to effectively filter all entering light and allow only a specific, variable wavelength to exit and reach the detector. So, the monochromator ensures that the only light that reaches the detector is the wavelength of light that is being absorbed by the sample being analyzed.

The other type of background that must be corrected for is non-specific absorption (and will be discussed in this chapter). This correction cannot be accomplished by the monochromator, since it involves the same analytical wavelength that is selected by the monochromator. Non-specific absorption (also called background absorption) has a broadband effect and occurs when the source light is prevented from reaching the detector by means other than absorption by analyte atoms, such as scattering and blocking of light by other species in the light path. Molecular species and solid particles present in the flame are the major causes of non-specific absorption. When the heat of the flame is not sufficient to fully break down all molecular species (for example, matrix compounds), there can be sufficient remaining molecules to absorb, block, or scatter the source light. These molecules can be thought of as causing the same interference as putting one’s hand in the light path: the light gets blocked, less light reaches the detector, and a falsely high absorbance signal results (because the signal analysis software “thinks” that less light is reaching the detector because more light is being absorbed by the analyte atoms). In FAAS, the background absorption is relatively minor (usually less than 0.05 absorbance units). In GFAAS, on the other hand, background absorption is severe and can reach levels of 2.0 absorbance units. Therefore, effective background correction methods become essential for accurate GF analyses. There are three background correction methods currently being used by AAS instrument manufacturers: Deuterium (D2), Smith-Hieftje (S-H), and Zeeman. For all three methods, there are several common factors that determine the effectiveness of the method:

• The frequency at which the peaks are measured; • The interval between total absorbance and background absorbance measurements; • The mathematical function used to calculate net atomic absorption; • The ability to correct for spectral or structured background; • The effect of the method on the linear working range.

The Frequency of Measurement

How rapid and transient the absorption peaks are dictates how fast absorption measurements must be made [9, 10].

In GF analyses, signal durations are typically 0.2 to 0.5 seconds. The temporally uniform, isothermal atomization of the AI 1200 provides peak durations that are in the low end of this range. The non-specific absorbance normally exhibits similar peak durations. Furthermore, the change in the absorbance values of these peaks can often be as high as 10 absorbance units per second. Therefore, a very high sampling frequency is required to accurately measure the very rapid and narrow peaks and to determine peak shapes.

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For a GFAAS system, a 10 Hz sampling frequency can produce significant errors in the measurement of peak height and/or peak area. While the performance can be improved if a 30 Hz frequency is used, the errors still remain significant. An increase to a 60 Hz sampling frequency will provide acceptable results in a GFAAS system. The AI 1200 utilizes a sampling frequency as high as 1000Hz in single beam mode and 120 Hz in double mode, giving excellent peak definition and negligible errors in peak area and peak height measurements.

The Interval between Measurements The Net Absorbance Signal (NAS) is the analytical signal of interest. NAS is the absorption of the light that is attributed only to the analyte atoms. It is obtained by subtracting the Background Absorbance Signal (BAS) from the Total Absorbance Signal (TAS).

Ideally, if BAS and TAS were measured simultaneously, then there would be no error in the NAS calculation. In reality, however, only one measurement can be made at a time. The best thing to do, then, is to make the time interval between successive measurements as short as possible, to approach simultaneous measurements. The shorter the time interval between a BAS measurement and a TAS measurement, the less error there will be in the calculation of NAS. This factor is especially important for GFAAS, where the background signal can change quite rapidly compared to the analyte signal.

The AI1200, which uses the D2 background correction method, uses a D2/HCL modulation frequency of 1 KHz, resulting in a time interval between successive measurements of less than 0.5 ms. With this system, the HCL and D2 lamps are alternately pulsed so that only one light source is passing through the sample at a time. When the HCL is pulsed, the D2 lamp is turned off and only the HCL light passes through the sample. The signal measured is the TAS. When the D2 lamp is pulsed, the HCL is turned off and only the D2 light passes through the sample. The signal measured is the BAS.

The Function used to Calculate Net Absorption If a sufficiently fast switching frequency between the BAS and TAS measurements is employed (such as in the AI 1200), then a simple subtraction of the BAS from the TAS can be performed to obtain the NAS. If relatively long time periods are elapsed between the BAS and TAS measurements, then interpolation techniques will be needed to approximate the NAS values. Furthermore, there are increased chances of significant changes in the background signals occurring over the elapsed time periods. Therefore, these interpolation techniques are much more susceptible to errors and inaccurate NAS values.

Spectral and Structured Background Less than 1% of the samples encountered in the real world exhibit spectral or structured background interferences that cannot be overcome by optimizing the atomizer (for GFAAS, in particular, by optimizing the furnace heating program) and/or using an appropriate chemical modifier.

The Effect on the Linear Working Range Due to complex splitting patterns, the TAS response of an instrument equipped with Zeeman background correction can be non-linear. As a result, when the BAS is subtracted from the TAS, a roll over point on a calibration curve can occur. That is, two different concentration values could correspond to a single NAS value. In such a case, the linear dynamic range has been reduced by the effects of the background correction method. In comparison, the D2 background correction method does not produce a roll over point in calibration curves and so has no negative effects on the linear dynamic range.

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AI 1200 Cookbook - Book One 3 - Background Correction Methods Deuterium (D2) Background Correction The light source used in the Deuterium (D2) background correction method is the D2 lamp. The D2 lamp is a continuum source, rather than a sharp line emitter. That is, it emits a spectrum of radiation covering 180 nm to 425 nm. Even though this means that the use of the D2 method is limited to the UV range, it is not much of a detriment since the most significant background absorptions occur at low wavelengths anyway. See figure 3.1 for a graphical explanation of how D2 background correction works. The key thing about D2 background correction is the assumption that the absorption of radiation by the analyte atoms alone has a negligible effect on the D2 spectrum. The monochromator exit slit is relatively wide (at least 0.2 nm), so a spectrum of D2 light of that width is allowed through. Compare this to the tiny width of spectrum that the analyte absorption occurs in (approximately 0.002 nm), and it’s easy to see how absorption occurring in such small region of a wide spectrum will have a negligible effect on that spectrum’s overall intensity. In other words, only a small fraction of the D2 spectrum is attenuated by the analyte absorption, and the rest is completely unaffected. Conversely, the atomic absorption due the analyte atoms has a significant effect on the intensity of the HCL radiation, since the HCL line is very sharp and narrow to begin with. But the absorption that occurs in real-life experiments is not due to analyte atoms alone. It is always a combination of analyte absorption and background absorption. Background absorption is broadband and so has an equal effect on the intensity of both the HCL line and the D2 band. So, when the individual effects of the analyte absorption and the background absorption are added together, the result is that the HCL line is attenuated cumulatively by both and the D2 band is only attenuated by the broadband background absorption. This fact forms the foundation of the D2 background correction method. When the D2 lamp is pulsed, the absorption of the D2 band is measured (the BAS). When the HCL lamp is pulsed, the absorption of the HCL line is measured (the TAS). The difference between the two (TAS - BAS) is the absorption due to the analyte atoms alone (the NAS), and this is the figure of analytical significance.

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Figure 3-1 How D2 background correction works (continued on next page)

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Figure 3-1 (continued) How D2 background correction works

In an AAS instrument with D2 background correction, the optics will have to be constructed to accommodate the additional D2 lamp, and they must also be designed to ensure that the D2 light and the HCL light follow exactly the same path through the optics system. In order to achieve accurate and reliable background correction, it is imperative that the two light sources follow exactly the same path. If they did not, then the background signal measured with the D2 lamp will have no relevance to the total absorption signal measured by the HCL lamp. Imagine if the D2 light path went through the top of a flame or graphite tube, and the HCL light path went through the bottom of flame or graphite tube. In such a case, it cannot be assumed that the background in one area is the same as the background in another. Unless both the background and total absorption signals are measured along exactly the same path, then no net analyte absorption signal can be determined. Today, instrument optics are easily manufactured with enough precision and with adequate means of optimization to virtually eliminate all problems due to poor optical path alignment.

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AI 1200 Cookbook - Book One 3 - Background Correction Methods Smith-Hieftje (S-H) Background Correction The Smith-Hieftje background correction method is based on the phenomenon of self absorption (or self reversal) that occurs when HCLs are operated at very high currents [11]. When an HCL is operated at a very high current, the atoms that are sputtered inside the HCL can sometimes absorb the radiation that is emitted by other atoms inside the HCL. This causes the emission line to be broadened and the intensity of the emission line at the resonance wavelength to be decreased. The resulting emission profile looks like a broad, two-headed peak. The two heads sit on either side of the resonance wavelength, and the valley between the peaks is right at the resonance wavelength (the atomic absorption wavelength). Figure 3.2 shows emission profiles for an HCL line at different currents. At low current, the line is sharp and narrow. As the current is increased, the line gets more intense but it also broadens. At very high currents, self absorption occurs within the HCL, and the two-headed peak straddling the resonance wavelength is observed.

Figure 3-2 The principle behind the Smith-Hieftje background correction technique

When the S-H method is used, the HCL is alternately pulsed between normal operating currents (where no significant self absorption occurs) and high intensity currents. The duration of the HI pulses is very short compared to the normal current pulses. During the period of the normal pulses, the analyte absorbance signal is measured. During the period of the HI pulse, the background absorbance signal is measured. With the S-H method, the profile of the emission from a HCL is semi-broadband (in that the emission has been broadened to an extent) and the intensity of the analytical wavelength (the valley between the shoulders) has been reduced. So, during the HI pulse, the wavelengths above and below the analytical wavelength (the shoulder wavelengths) are used to measure the background absorbance. The attenuation of the already-diminished analytical wavelength by analyte atoms is negligible compared to the attenuation of the shoulder wavelengths by the non-specific absorbing species. So, the signal measured during the HI pulse is the background signal (BAS) Of course, during the normal pulses, the HCL emits the same sharp line as for the D2 background correction method. So, during the normal pulse of the HCL, the total absorption signal (TAS) is measured. A key advantage of S-H background correction method, in terms of accuracy, is that only one lamp is needed. Since a single lamp acts as both the “continuum” source and the sharp line emitter, there will never be any problems with the two optical paths not being exactly the same.

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AI 1200 Cookbook - Book One 3 - Background Correction Methods However, there is also a disadvantage of the method related to accuracy. Since the shoulder wavelengths of the HI pulse are used to measure the background (and not the analytical wavelength itself), there is some degree of approximation going on when that background measurement is subtracted from the total absorbance signal (for which the analytical wavelength is used). The assumption is that the background doesn’t change much in the short spectral width between the two shoulder wavelengths. In practice, this assumption is adequate. There are several other disadvantages to the S-H background correction method. While the requirement of a special HCL (designed for S-H background correction) can’t really be called a disadvantage in itself (since all background correction methods require some kind of additional equipment), the fact that special HCLs are not available for all elements limits the method’s usefulness. Also, it is required that the high and low currents of the HCL be stabilized before any measurements can be made. This results in a typical sensitivity loss of 50%. Calibration curves can sometimes exhibit roll over points at higher sample concentrations, so this decreases the linear dynamic range of analysis. Also, the S-H method cannot correct for any structured or spectral interferences. Finally, the method is limited to relatively low modulation frequencies (normal 10 Hz), so is incompatible with the rapid transient signals common to GFAAS experiments. Zeeman Background Correction This background correction method is based on the splitting of spectral lines that occurs in magnetic fields [12]. In the presence of a magnetic field, an emission line from an HCL will be split into pi (π) and sigma (σ) lines. The original resonance line becomes the pi line and remains at the resonance wavelength. There are two sigma lines and they are shifted from the pi line (one above and one below) by thousandths of a nanometer. The exact amount of the sigma lines shift depends on the strength of the magnetic field. An important part of this background correction method is that the sigma lines are perpendicular to the applied magnetic field, and the pi line is parallel. So, when a polarizer is used to filter the light beam, the pi line can be excluded.

This method is effective because the species that are in most cases to blame for non-specific absorption (molecules and solid particles) are not affected by the magnetic field. So, when the magnetic field is applied, an accurate background absorption signal (BAS) can be measured since the non-specific absorbing species remain unaffected and the analytical resonance wavelength can be blocked by polarization. Of course, when the magnetic field is not applied, the measurement made will simply be of the total absorption signal (TAS). Again, the net absorption signal (the NAS, the absorption due to the analyte atoms alone) is determined by the difference between the TAS and the BAS. A key advantage to the Zeeman background correction method is that both the TAS and BAS are measured along the exact same optical path, so the technique is very accurate in that sense. As well, Zeeman-equipped instruments generally operate at a high frequency of measurement (usually 120 Hz), so they are able to deal with the rapid transient peaks that occur in GFAAS analyses. The Zeeman method is applicable to any wavelength of light, so it works for all elements. This is in contrast to both the D2 and S-H methods. The D2 method is limited to those elements with analytical wavelengths that fall in the range 180 - 425 nm, and the S-H is limited by the availability of a HCL for the element of interest. Furthermore, the Zeeman method is the only one that can correct for spectral and structured background interferences. While that last advantage listed may seem impressive, it is in reality of no consequence. Less than 1% of samples that are run in labs today have problems with spectral or structured background interferences that cannot be overcome with optimization of the atomizer or by the addition of suitable chemical modifiers. There are other drawbacks of the Zeeman background correction method that should be mentioned. There is typically a sensitivity loss of 20% associated with the Zeeman method (the exact value is determined by the magnetic susceptibility ratio of the

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AI 1200 Cookbook - Book One 3 - Background Correction Methods element being determined). Also, it frequently results in roll over of calibration curves at higher concentrations, so the linear dynamic range is significantly reduced. Another important point relates to the cost of the Zeeman instrumentation, which requires a relatively large initial investment. This is a result of all the additional equipment that is required on an AAS instrument equipped with Zeeman background correction (a stable electromagnet, a power supply, and a polarizer). Comparison of Background Correction Methods An important part of the decision to purchase an AAS instrument is the decision of which background correction method to include. In general, this decision is governed by the applications that the instrument is expected to be used for. Of course, there are specific applications that each background correction method is the best suited for. But it is quite rare that an instrument will be dedicated for a single, specific type of analysis throughout its entire lifetime. With that in mind, perhaps it is wisest to choose a background correction method that is versatile and can meet the background correction needs of the majority of samples encountered. Table 3.1 below is a summary of the comparisons between the three background correction methods discussed here. Table 3.1: Summary Comparison of Background Correction Methods

D2 S-H Zeeman Sensitivity Good Poor, 50% loss Moderate, 20% loss Type of Instrument All FAAS GFAAS Wavelength Range 180-425 nm 180-900 nm 180-900 nm Number of Elements Limited to elements in

wavelength range Limited to lamp

availability All

Longevity of Lamp Normal Decreased Normal Calibration Linearity Not affected Roll over occurs Roll over occurs Linear Dynamic Range Not affected Decreased Decreased Accuracy Two optical paths

must be optimized Good (single optical path)

Good (single optical path)

Correct for Spectral or Structured Interferences?

No No Yes

Frequency of Measurement 120 Hz 10 Hz 60-120 Hz Interval between Measurements

0.5 ms 2-4 ms 4.5 ms

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AI 1200 Cookbook - Book One 3 - Background Correction Methods D2 seems to be the overall best background correction method of all the three, since it offers the broadest range of advantages and does not suffer in the really key areas of sensitivity and calibration linearity. There seems to be an impression in the analytical chemistry community that Zeeman background correction is the superior method available. However, once the factors have all been weighed, it should become apparent that this impression is flawed. As was mentioned before, the fact that Zeeman method is the only one that can correct for spectral and structured background interferences is of no consequence in over 99% of the samples that will be encountered in a laboratory. Even if it is determined that this feature is necessary and Zeeman background correction is required, there will be other important factors that will suffer. Initially, the cost of the Zeeman instrumentation is very high compared to D2 and S-H. Also, Zeeman has significantly poorer sensitivity, calibration linearity, and linear dynamic range than the D2 method offers. Another misconception is that D2 background correction is inadequate for samples with rapidly changing background signals. The ‘frequency of measurement’ and ‘interval between measurements’ characteristics show that D2 is on par or better than Zeeman is in these areas. Based on all these considerations, the decision between the D2 and Zeeman background correction methods can be boiled down to the following:

Choose D2 if you need:

Choose Zeeman if:

• maximum sensitivity • calibration linearity • maximum linear dynamic range • rapid measurement capability • low cost

• you expect spectral and

structured background interferences, and this overrides all other factors

• cost of equipment is no concern

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AI 1200 Cookbook - Book One 4 - Comparison of AAS Techniques

Chapter 4: Comparison of AAS Techniques

• Things to Consider • Applications • Expected Concentration Ranges • Elements • Atomization Efficiency • Interferences

• Spectral • Background • Matrix

• Detection Limits • Sensitivity • Precision • Linear Working Range • Minimum Sample Volume • Sample Throughput • Sample Usage • Total Dissolved Solids • Method Development • Ease of Use • Automation/Unattended Operation • Costs

• Initial Investment • Running Costs

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AI 1200 Cookbook - Book One 4 - Comparison of AAS Techniques Things to Consider

When deciding whether to purchase a flame AAS or a graphite furnace AAS, one should first make an assessment of one’s analytical needs:

• Which elements will I be analyzing? • What are the sample matrices? • What are the solvents? • How much sample volume will I have? • Do I need high throughput? • Will the concentrations be high or at trace levels? • What detection limits do I need? • How important are precision and sensitivity? • How skilled are the intended operators? • Can operators afford to spend time on method development and optimization? • How much money am I willing to spend?

Applications The applications that you will be running (e.g. environmental, marine, petroleum, mining) will play a part in determining the AAS technique that is right for you. However, many of the techniques overlap when they are compared on the basis of applications. Therefore, your particular application will be a relatively minor factor in the decision. That aside, there are some applications that one technique is much better suited for than the other (this is usually based on the element(s) being analyzed). It’s also important when deciding on a AAS instrument to think of the future. Will your anticipated long-term needs be satisfied by the instrument that is appropriate to purchase for your needs today? Expected Concentration Ranges Do you expect most of the samples that you will analyze to have high or low concentrations? Since GFAAS is much more sensitive than FAAS (typically, GFAAS has detection limits 100 to 1000 times better), it makes more sense to run the concentrated samples on a FAAS, and use a GFAAS to run the trace analyses. Diluting samples down to concentration levels that a GFAAS can handle is not only time consuming and labor intensive, it also introduces the possibilities of errors and contamination. Elements The FAAS technique can analyze a total of 61 elements with its air/acetylene and nitrous oxide/acetylene flames. The GFAAS technique can analyze a total of 41 elements. Also, any element that GFAAS can analyze can also be analyzed with FAAS. Atomization Efficiency

The atomization efficiency is arguably the most important performance

characteristic of any AAS technique. The air/acetylene flame burns at around 2300 °C, and the nitrous oxide/acetylene flame burns significantly hotter at around 3000 °C. For some elements, the cooler air flame is not hot enough to atomize the sample, and the nitrous oxide flame is required instead. However, the excess energy of the hotter nitrous oxide flame can ionize other elements. So, it’s often a trade-off with air flame atomizers. Some elements have low atomization energies, and so are better suited to the cooler flame

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AI 1200 Cookbook - Book One 4 - Comparison of AAS Techniques (copper, iron, nickel, and gold). Others have high atomization energies, and so a nitrous oxide flame is needed (aluminum, silicon, titanium, and tungsten). And for some elements, no flame is hot enough to efficiently atomize the sample. In these cases, the GFAAS technique is needed, because the furnace can reach temperatures up to 3000 °C. Another consideration is the nebulization efficiency of the FAAS nebulizer. Only less 10% of original sample introduced into the spray chamber gets converted in an aerosol and makes it into the flame for atomization. In this sense, the FAAS technique is very wasteful. The GFAAS technique, on the other hand, is very efficient. All of the sample deposited in the graphite furnace remains inside the furnace and therefore has a good chance of being atomized. As well, atomized sample have relatively long residence times inside the furnace tube (0.2 – 0.5 s) Interferences

Spectral Spectral interferences do occur, but are rare, in both FAAS and GFAAS techniques.

Background

Background interferences are much more common, and both techniques are vulnerable. Background correction techniques are usually employed to compensate for the interference. In GFAAS, background interference typically arises from vaporized and atomized matrix components.

Matrix Matrix interferences can be serious for both FAAS and GFAAS analyses. For GFAAS, the effects can be countered by adding modifiers to the samples. For FAAS, the ionization buffers are usually employed to control matrix effects related to ionization. Detection Limits

The detection limits of the GFAAS technique (ppb range) are generally 100-1000

times better than the FAAS technique (ppm range). Sensitivity This factor is closely linked to detection limits. GFAAS is much more sensitive than FAAS, since the graphite furnace atomizer has much longer residence times and much higher atomization efficiency than the flame atomizer. Precision The flame atomizer has better short term precision (0.5%) than the graphite furnace atomizer (2-5%). Long term precision is usually better with the flame atomizer too, since the graphite furnace invariably degrades and gets worn out as it is used. Linear Working Range The linear working range of the AAS technique is extremely important. It is the concentration range over which standard samples can be measured and used to generate a linear calibration curve. Having a narrow working range means that the calibration curve’s usefulness is limited because not as many unknown samples will fall within the linear calibration range.

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AI 1200 Cookbook - Book One 4 - Comparison of AAS Techniques In general, on an element-by-element basis, the linear working ranges of the FAAS technique are better than those of the GFAAS technique. In this respect, FAAS is more versatile and useful than GFAAS. Minimum Sample Volume The minimum sample volume required to measure an absorption signal of a solution can be very important sometimes, such as when there is almost no sample solution to begin with. The minimum volume required by the GFAAS technique is much smaller than for the FAAS technique. GFAAS requires as low as 20 µL of sample for a good measurement, whereas FAAS requires at least 0.5 mL of solution. Sample Throughput The sample throughput is the rate at which individual samples can be analyzed. It is often expressed in the format minutes per sample. FAAS has a higher sample throughput rate than GFAAS. The reason for this is that the graphite furnace atomization sequence has multiple steps, each with hold times, and a cool-down time is required at the end of each analysis. Typical sample throughput values are 10 seconds per sample for FAAS analyses and 4 minutes per sample for GFAAS analyses. Sample Usage Sample usage refers to how much sample is consumed by the instrument while it performs an analysis. FAAS uses a lot of sample solution for each analysis, and much of it is wasted and never even makes it into the atomizer for an absorption measurement. GFAAS, on the other hand, uses very little sample and wastes none. The entire volume of sample deposited in a graphite furnace remains in the graphite furnace during the atomization steps. Total Dissolved Solids The graphite furnace atomizer can handle relatively large amounts of dissolved solids (> 20%), whereas the flame atomizer can only tolerate about 3% maximum (this is the case for liquid introduction atomizers in general). Method Development The FAAS is easier to develop methods for than the GFAAS instrument. This is mostly because of the strict constraints set on the optical alignment of the furnace atomizer and the requirement of accurate and consistent sample injections. As well, the operation of the FAAS doesn’t require as much experience as the operation of the GFAAS. Nonetheless, for both instruments, a skilled operator is required to develop a good method for a specific application. Fortunately, most instruments come equipped with extensive libraries of pre-made, common methods that are ready to use. Ease of use The FAAS instrument is very easy to use, and an expert operator is not required for routine analyses. The GFAAS instrument is also relatively simple and straight-forward to use, but a skilled operator is still required because accurate and consistent sample injections are essential for good results, even for the most routine analyses.

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AI 1200 Cookbook - Book One 4 - Comparison of AAS Techniques Automation/Unattended operation GFAAS lends itself well to automation. The use of inert gases does not pose any dangers, so unattended operation is also feasible. Overnight, automated operation of GFAAS is commonly employed to achieve a higher sample throughputs. FAAS is equally suited to automation, but unattended operation is unadvisable due to the use of combustible gases. An operator should always be present to ensure the safe operation of the instrument and to be able to deal with gas leaks or other emergencies as they arise. Costs

Initial Investment There is no way to specifically say how much an instrument will cost, since there are many factors involved. The cost depends on the configuration, accessories, options, background correction method, and, of course, the vendor. Roughly speaking, FAAS instruments can cost from $20,000 to $35,000. GFAAS instruments are more expensive, and usually cost about 2 times as much ($40,000 to $70,000).

Running Costs Both techniques are comparable in terms of the costs of running the instruments on a day to day basis, although FAAS is generally a cheaper instrument to run. There is a constant expense for the fuel and oxidant gases in FAAS (although, air is free), and the inert gas used in GFAAS (usually argon) isn’t cheap. In addition, a periodic expense of the GFAAS technique is the replacement cost of the graphite furnace atomization tube, which has a limited life and needs to be replaced from time to time to ensure efficient atomization. Another consideration is whether your specific applications with GFAAS will require clean room conditions. Maintaining clean room conditions is no small task and adds considerably to the operating costs of the instrument.

The following table is a summary of the comparisons between FAAS and GFAAS. Table 4.1: A Summary Comparison of Flame AAS and Graphite Furnace AAS

FAAS GFAAS Detection Limit sub ppm sub ppb Linear Working Range 103 102

Precision 0.5% 2 - 5% Sensitivity good excellent Spectral Interferences virtually none occasionally Chemical/Matrix Interferences many many Ionization occasionally minimal Minimum Sample Volume 0.5 mL 20 µL Sample Throughput ~10 s/sample ~4 min/sample Sample Usage high low Number of Elements 61 41 Maximum Dissolved Solids ~ 3% > 20% Ease of routine use easy requires expertise Method Development/Optimization easy difficult Easily Automated? moderately yes Combustible Gases? yes no Initial Investment low high Running Costs low moderate

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38

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39

AI 1200 Cookbook - Book One 5 - Standards and Sample Preparation

Chapter 5: Standards and Sample Preparation

• Apparatus • Water • Standard and Blank

Solutions • Sample Solutions • Storage of Solutions • Calibrations • Matrix Effects • Chemical Interferences

• Incomplete Dissociation of Analyte Compounds

• Ionization

40

AI 1200 Cookbook - Book One 5 - Standards and Sample Preparation Apparatus All apparatus to be used during the process of an analysis, from the sample and standard preparation to the introduction into the instrument, must be scrupulously clean. Even glassware that had been cleaned previously and stored should be cleaned again immediately before use. As well, the instrument itself should be periodically cleaned to ensure that no contamination errors or memory effects are introduced. In FAAS instruments, the sample aspiration tubing is easily and inexpensively replaced. The nebulizer chamber should also be cleaned and washed regularly. Residue buildup can be a problem, especially when flow spoilers or baffles are employed. Corrosion resistant nebulizer should be used when corrosive acids or organics are being analyzed (e.g. use a Teflon constructed spray chamber and glass impact bead). Volumetric flasks should be Class A (accurate to ± 0.01%), since Class B glassware will introduce systematic measurement errors into your analysis. Analytical balances should be high-accuracy and high-precision. Less than 1% error is needed when weighing out samples to ensure minimal error in prepared solutions. Pipettes and micropipettes should always be calibrated to ensure accuracy of delivery volumes. Water All water used for AAS analyses should be deionized. Distilled water alone is likely to have significant amounts of dissolved metals and gases, and can introduce significant error into analyses. Using distilled water that has also been deionized is acceptable. For some GF applications, particularly when trace levels are being determined, simple deionized water is not good enough. In these cases, an ultra-pure water treatment is needed (eg. the Millipore system). In general, one can expect most acids to contain trace amounts of metallic impurities. While the levels are trace, they can still introduce error in some applications (particularly in trace-analyses and GF analyses). Fortunately, specially purified acids are commercially available to alleviate this source of error. Standard and Blank Solutions Bulk standard solutions are available from many chemical suppliers, usually in 1000 mg/L (1000 ppm) concentrations. They have a shelf life of about one year. These bulk standards come with certificates of analysis, and are traceable to NIST (National Institute of Standards and Technology) standards. These bulk standards should be of the highest purity available. Atomic Absorption grade is acceptable for many applications, but Ultra-pure is preferred (and essential for very sensitive, trace-level determinations). Bulk standard solutions can also be prepared by the operator, in the laboratory. This method is useful to prepare standards from pure metals, metallic salts, and metal oxides. When weighing out the raw materials, the materials must be treated to ensure that they are in a pure, standard form. Metals should be washed with acetone or ether to remove any oil layers. Oxide coatings can be removed by scrubbing them off the metal’s surface (with an emery cloth, for example). Metal oxides, salts, and other compounds should be dried for several hours at an elevated temperature to drive off excess water (110 °C or higher will suffice). The material should be dried to a constant mass. In the interest of minimizing errors, it is best to perform serial dilutions when performing large dilutions of the bulk standards. For example, imagine that you needed to prepare a 0.1 ppm standard from the 1000 ppm bulk. This is a dilution factor 10,000. Rather than performing such a dilution in one step, it is advisable to break it into two steps: perform a 100 times dilution, and then another 100 times dilution. The end result

41

AI 1200 Cookbook - Book One 5 - Standards and Sample Preparation is the same, but the chance for error is much less, since a small volume measurement error has a relatively greater effect on a small volume than on a larger volume. However, the error in volume measurements that do occur in the two step method are doubled, since the dilution is performed twice. However, the relative effect of the volume measurement error is of far greater concern. In this example, the two step dilution uses volumes of the bulk standard that are 100 times greater than volumes used with the single dilution method. Therefore, the two step dilution method, in theory, has 50 times less error.

Ideal standard solutions are solutions that are identical in chemical and physical composition to the sample solutions, except that they have known concentrations of the analytical component. Ideal blank solutions are solutions that are identical in chemical and physical composition to the sample solution, except that they don’t have any of the component that is of analytical interest. Proper standards and blanks should be prepared by the same procedure that the sample is prepared by. For example, if the sample is microwave digested by nitric acid, then the standards and blanks should also be microwave digested by nitric acid (expect that the standards will have a known amount of the analytical component added, and the blanks will have none added).

Matching the physical properties of sample with standards and blanks is important because the amount of solution that is aspirated by the nebulizer and into the flame depends, to an extent, on the physical properties of the solution. The viscosity, density, surface tension, and solvent vapor pressure will all affect the aspiration rate. Sample Solutions There are many different kinds of sample solutions. Sometimes the sample is obtained from a third party, and is already in ready-to-analyze form. Often, however, digestion and dissolution steps are required. In either case, it is important that sample solutions be homogenous and free of solids. This is especially important for the GFAAS analyses, since the sample volumes analyzed are much smaller than for FAAS. If 50 µL of a heterogeneous sample is deposited in the graphite furnace and analyzed, it is very likely that its signal will be different from the next 50 µL sample analyzed, producing inconsistent results. In other words, it is very difficult to get a representative sample aliquot from a heterogeneous sample solution. Storage of Solutions It is recommended that all working standard solutions to be used in an analysis be prepared (from the bulk standards) fresh on the same day as the analysis. All solutions should be labeled and dated to keep track of them. This is to prevent any errors due to the standard solution ions absorbing into the container walls, and also molecules leaching from the container walls into the sample solution. The same applies to sample solutions. Bulk solutions stored on the shelf for long periods should have high concentrations (at least 1000 ppm). There is significant degradation solutions with low concentrations (less than 10 ppm), mostly from adsorption onto the container walls, so it is imperative that they be prepared fresh and utilized promptly. Additionally, when storing solutions, it is good practice to use polyethylene containers instead of glass vessels. This is because some metal ions have an affinity for glass walls and will become adsorbed onto glass surfaces. A common and convenient way to prolong the shelf life of working samples and standards is to acidify the solutions with mineral acids to a pH of about 2. Nitric acid (HNO3) is most frequently used.

42

AI 1200 Cookbook - Book One 5 - Standards and Sample Preparation Calibrations The method of calibration used in an analysis depends almost entirely on the chemical and physical properties of the unknown sample. For example, if the sample solution has a very simple aqueous matrix, and its properties and behavior in the atomizer are well known, then a normal calibration will be sufficient. On the opposite end of the spectrum, if the sample solution has a complex matrix and the chemical and physical properties are not all known, then a standard addition calibration method will be required. The former case can be divided into two sub categories: samples with simple, known matrices, and samples with complex, known matrices. In both cases, the matrices are reproducible in the standards and samples, and so a normal calibration is sufficient. An example of a simple, known matrix is a sample that has an unknown amount of copper dissolved in nitric acid and water. An example of a complex, known matrix is a sample that has an unknown amount of copper from a steel sample. This sample has a complex matrix because there are other elements present (eg. iron, nickel, aluminum, and chromium). The matrix is known because the amount of other elements present is roughly known. The latter case can also be divided into two sub categories: complex, unknown matrices, and organic matrices. In both cases, the matrix is not reproducible in the standards and blanks, so the differences between the signals of the sample and standards would be due to more factors than just the analytical component. An example of a complex, unknown sample matrix is a sample with copper that was derived from raw oyster meat by mechanical grinding, two-stage acid dissolution, and then furnace heating and drying. While all the digestion steps could be reproduced in the standards and blanks, the added matrix components from the oyster meat could not. To accurately analyze samples with unknown matrices, a standard addition calibration method is required. This usually means preparing a series of “standards” that are prepared from samples of the unknown that have different-sized aliquots of the analyte element added to them. The standards prepared in this way all have a matrix that is approximately the same as the unknown sample, since all the standards were prepared from the unknown sample. The calibration curve is generated by first plotting absorbance versus “amount of standard added to unknown”. The “amount” can be expressed as volume, mass, moles, or whatever unit is appropriate. So, when the unknown is analyzed without any standard added, the absorbance value is plotted at x = 0 mL. When the unknown is analyzed with, for example, 0.5 mL of standard added, the absorbance is plotted at x = 0.5 mL. A calibration curve is generated by continuing on in this manner. Extrapolation of the curve to intersect the x-axis gives the concentration of analyte element in the original unknown sample. Matrix Effects The precipitation of the element of interest from working solutions is a common cause of inaccurate analyses. This is common, for example, with tin and silicon solutions. Chemical Interferences The technique of AAS is based on the formation of ground state atoms being formed in the atomizer, thereby making them capable of absorbing light. The usefulness of AAS can be severely compromised when the formation of ground state atoms is inhibited in any way. Chemical interference is one cause of such inhibition. Chemical interference is a result of either of two phenomena:

(a) incomplete dissociation of analyte compounds (b) ionization of analyte atoms

43


Incomplete Dissociation of Analyte Compounds In general, this kind of chemical interference arises when refractory compounds of

the element of interest are introduced into the flame (or are formed in the flame). Refractory compounds are not easily broken down by the energy of flame atomizers, and therefore the atomizer is inefficient in producing neutral ground state atoms of the element of interest.

A common way to prevent this type of chemical interference is to add an excess of some other kind of element to the solutions. The refractory compounds must have a greater affinity to form with the added element than with the element of interest. Thus, the element of interest is freed up and the atomizer will be more efficient in producing ground state atoms.

Ionization Ionization of analyte atoms occurs when the energy of the atomizer is too high and

some of the ground state neutral atoms formed by the atomizer are actually ionized by the high energy environment. Some elements are more prone to this than others. The alkali and alkali earth elements, for example, are known as “easily ionizable elements”.

A common remedy for analyte ionization is to add an excess of another element whose ionization potential is lower than the element of interest. When an excess of the easily ionizable element is added, it creates an excess of electrons in the atomizer, and thus shifts the ionization equilibrium to effectively eliminate ionization of the element of interest.

44



45



46

AI 1200 Cookbook - Book One 6 - FAAS Analytical Data Sheets

Chapter 6: FAAS Analytical Data Sheets

Introduction:

How to Use These Analytical Data Sheets The following Analytical Data section is the heart of this cookbook. It has guideline methods for each individual element. It is important to realize that the parameters listed are only meant as guidelines. The parameter values listed will not necessarily lead to the best performance on every instrument. Rather, they are to serve as a starting point for developing optimized methods on an instrument-by-instrument basis.

For example, on page 56 is the analytical data for analyzing cadmium by FAAS. The parameter PMT voltage is listed as 375 V. This does not mean that 375 V will be the right voltage for every instrument. In fact, it’s very unlikely that it ever will be. Rather, the 375 V value is meant as a ballpark figure; it indicates that the PMT voltage of an optimized method for measuring cadmium with FAAS should be around 375 V. This information is valuable to the operators creating the method because they have a general idea before hand of what the PMT voltage should be. When the operator creates a method for cadmium, they can start at 375 V and make adjustments from there. This is much easier and quicker than guessing at a starting point. The operator could guess to start at 100 V, which would definitely be too low. Or the operator could guess to start at 600 V, which would definitely be too high. In summary, the parameter values listed in the Analytical Data sheets are meant as guidelines, or starting points, for creating optimized methods. They are not meant as exact values that could be “cut-and-paste” into blank methods to produce perfect, optimized methods every time.

There are a few things that the operator should know when looking at the ‘Guideline Parameters’ table of the data sheets. An oxidizing flame is fuel lean (relatively low ratio of fuel to air) and is blue in color. A reducing flame is fuel rich (relatively high ratio of fuel to air) and is orange or red in color. A stoichiometric (or neutral) flame has a fuel to air ratio of one. The characteristic concentration of an element is the concentration of analyzed sample that would produce an absorption signal of 0.0044 Abs, or 1% absorption. The working range is the concentration range within which a reasonably linear calibration curve can be constructed. Excessive curvature of the calibration curve occurs outside of this range. Some graphs are included on the element data sheets if the absorption sensitivity of the element is sensitive to the parameter in the graph. For example, a graph of absorbance vs. lamp current is included on the data sheet for silver because the absorption signal depends heavily on the lamp current. The graph gives the operator a general idea of what kind of absorption changes to expect when the parameter is varied. Information is provided on how to prepare standard solutions for each element. Alternatively, certified standards can be purchased from chemical suppliers.

47


Information about common interferences is also provided for operator reference. It would be wise to become familiar with these sections before analyzing an element. With prior knowledge of interferences, major inconveniences and errors can be avoided. The data sheets for the elements are arranged alphabetically by element name (not by element symbol). For reference, the atomic number of each element is displayed in a box along with the element symbol.

48


13

Al

Aluminum

Analytical lines: 309.3 nm 396.2 nm

Guideline Parameters - FAAS

Parameter

Flame: nitrous oxide/acetyleneFlame Stoichiometry: reducing HCL Current: 5.0 mA PMT Voltage: 375 V Slit Width: 0.2 nm Characteristic Concentration:

1.0 mg/L

Working Range: 0.3 - 100 mg/L

Absorption Sensitivity

0

0.2

0.4

0.6

0.8

1

0 25 50 75 100

Concentration (mg/L)Abso

rban

ce

Standard Solutions Dissolve 1.000 g of Al metal wire in 20 mL of hydrochloric acid. Add a drop of mercury salt catalyst. Dilute to 1 L with 1% (v/v) HCl to give a 1000 ppm Al solution. Retrieve the mercury by filtering the solution.

Interferences Aluminum is prone to ionization in the nitrous oxide flame. Add an easily ionizable element (K or La, 0.1%) to alleviate this interference. Absorption is depressed by silicon, calcium, and phosphate. Absorption is increased by iron, titanium, nickel, and manganese.

49


51

Sb

Antimony



Parameter

Flame: air/acetylene Flame Stoichiometry: oxidizing HCL Current: 10 mA PMT Voltage: 460 V Slit Width: 0.2 nm Characteristic Concentration:

0.15 mg/L

Working Range: 0.1-30 mg/L

Absorption Sensitivity (217.6nm upper; 231.2nm lower line)

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40Concentration (mg/L)

Abso

rbance

Standard Solutions Dissolve 1.000 g of antimony metal granules in 100 mL of HCl with 2 mL of HNO3. Dilute to 1 L to give a 1000 ppm Sb solution (oxidation state Sb(V)). All solutions made from this stock solution should be diluted with 10% HCl. Absorption Sensitivity vs. Fuel flow Rate

0

0.1

0.2

0.3

0.4

0.5

0.5 1 1.5 2 2.5 3 3.5Fuel Flow Rate (L/min)

Abso

rban

ce

OR Dissolve 2.743 g of K(SbO)C4H4O6 ½H2O (potassium antimonyl tartrate) in H2O. Dilute to 1 L to give a 1000 ppm Sb solution.

WARNING: This element is very toxic and should be handled with extreme caution. Personal protective equipment is essential.

Interferences High levels of acids cause a depression of the signal. Usually, matrix matching of solutions can compensate for this interference. Cu and Ni decrease the signal. This interference is more pronounced in reducing flames than in oxidizing ones.

50


33

As

Arsenic



Parameter

Flame: air/acetylene Flame Stoichiometry: reducing HCL Current: 20 mA PMT Voltage: 700 V Slit Width: 0.6 nm Characteristic Concentration:

0.6 mg/L

Working Range: 2 - 100 mg/L


0

0.2

0.4

0.6

0.8

1

0 40 80 120Concentration (mg/L)

Abso

rbance

Standard Solutions Dissolve 1.3203 g of arsenious oxide (As2O3) in a minimum volume of 20% (w/v) KOH. Neutralize with HNO3 to a phenolpthalein endpoint. Dilute to 1 L with 1% (v/v) HNO3 to give a 1000 ppm arsenic solution.

WARNING: Arsenic is toxic and should be handled with extreme care. Personal protective equipment is essential.

Absorption Sensitivity vs. Fuel flow Rate

0

0.1

0.2

0.3

0.4

1 1.5 2 2.5Fuel Flow Rate (L/min)

Abso

rban

ce

Interferences The flame itself absorbs or scatters more than half of the source light at 193.7nm, so background correction is highly advised. The 189nm line is generally not used because of heavy atmospheric absorption. Total salt content greater than 1% can also cause non-specific absorption.

51


56

Ba

Barium



Parameter

Flame: nitrous oxide/acetyleneFlame Stoichiometry: reducing HCL Current: 10 mA PMT Voltage: 381 V Slit Width: 0.2 nm Characteristic Concentration:

0.9 mg/L

Working Range: 0.4-150mg/L


0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

• Data collected with potassium chloride added (final concentration of 2g/L) to suppress ionization.

Standard Solutions Dissolve 1.7785 g of BaCl2 2H2O (barium chloride) in H2O. Dilute to 1 L to give a 1000 ppm Ba solution.

0

0.1

0.2

0.3

be fore a fte rAd dition o f 2 g/L KC L

Ab

so

rba

nce

OR Dissolve 1.437 g of BaCO3 (barium carbonate) in a minimum volume of 1:1 HCl. Dilute to 1 L with 1% HCl to give a 1000 ppm Ba solution.

Interferences Barium is partially ionized in the nitrous oxide flame. Addition of an alkali salt (eg. KCl, 0.1%) can suppress the effects. In samples with Ca, spectral interference arises from the 553.6nm CaOH line.

52


4

Be

Beryllium

Analytical lines: 234.9 nm


Parameter


0.012 mg/L



0

0.2

0.4

0.6

0.8

1

1.2


Abso

rbance

Standard Solutions Dissolve 1.000 g of beryllium metal wire in 1:1 HCl. Dilute to 1 L with 1% v/v HCl to give a 1000 ppm Be solution.

WARNING: This element is toxic and should be handled with extreme care. Personal protection equipment is essential.


0.1

0.2

0.3

0.4

0.5

3.5 4.5 5.5 6.5Fuel Flow Rate (L/min)

Abso

rbance

Interferences Al, Mg, Na, and Si, when present at high levels, can decrease the Be signal. The interference due to Al can be reduced by the addition of 1% HF. The interference due to Mg and Si can be reduced by adding oxine.

53


83

Bi

Bismuth

Analytical lines: 222.8 nm 306.8 nm 206.2 nm 227.7 nm


Parameter


0.19 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

• A higher HCL current provides a more stable absorption signal.

Standard Solutions Dissolve 1.000 g of Bi metal wire in a minimum volume of 1:1 HNO3. Dilute to 1 L with 2% HNO3 to give a 1000 ppm Bi solution.

Interferences Absorption Sensitivity vs. Slit Width

0

0.1

0.2

0.3

0 0.2 0.4 0.6 0.8Slit Width (nm)

Abso

rban

ce

No major interferences to note.

54


5

B

Boron



Parameter


14 mg/L

Working Range: 8-3000 mg/L


0

0.2

0.4

0.6

0.8

1


Abso

rbance

Standard Solutions Dissolve 44.095 g of Na2B4O7 10H2O (sodium borate) in H2O. Dilute to 1 L to give a 5000 ppm B solution. OR Dissolve 28.60 g of H3BO3 (boric acid), in H2O. Dilute to 1 L to give a 5000 ppm B solution.

Absorption Sensitivity vs. Slit Width

0

0.1

0.2

0.3

0.4

0 0.2 0.4 0.6 0.8Slit Width (nm)

Abso

rban

ce

Store solutions in polyethylene bottles.

Interferences Boron is the one of the least sensitive elements in AAS due to the formation of stable oxides, nitrides and carbides. The sensitivity can be improved by converting it to volatile boric acid methyl ester or to volatile fluoride by reaction with copper hydroxyfluoride. High levels of sodium can cause interference, which can be alleviated by using a stoichiometric flame. However, sensitivity is sacrificed.

55


48

Cd

Cadmium



Parameter

Flame: air/acetylene Flame Stoichiometry: oxidizing HCL Current: 5.0 mA PMT Voltage: 375 V Slit Width: 0.2 nm Characteristic Concentration:

0.01 mg/L

Working Range: 0.02 - 2.5 mg/L

Absorbance vs. Concentration

0

0.2

0.4

0.6

0.8

1

0 1 2Concentration (ppm)

Abso

rban

ce

Standard Solutions Absorbance vs. Fuel flow rate

0.4

0.41

0.42

0.43

0.44

0.4 0.6 0.8 1 1.2 1.4 1.6Fuel Flow Rate (L/min)

Abso

rban

ceDissolve 1.000 g of Cd metal strip in a minimum volume of 1:1 HNO3. Dilute to 1 L with 1% HNO3 to give a 1000 ppm Cd solution.

WARNING: Cadmium is a toxic element and should be handled with extreme care. Personal protective equipment is essential.

Interferences No major interferences to note.

56


20

Ca

Calcium



Parameter


0.066 mg/L


A bso rption Sen s itiv ity

0

0.2

0.4

0.6

0.8

1

0 3 6 9 12 1 5C once ntra tion (m g /L)

Ab

so

rba

nce

• The absorption sensitivity is very dependent on the fuel flow rate and burner head position.

• The best precision is achieved with an oxidizing flame, and best sensitivity with a reducing flame.

• A wide slit width can be used for determinations in the air flame, but is not recommended for the nitrous oxide flame. Absorption Sensitivity vs. Fuel flow Rate

0

0.05

0.1

0.15

0.5 1 1.5 2 2.5Fuel Flow Rate (L/min)

Abso

rbance

Standard Solutions Add 2.497 g of dried CaCO3 (calcium carbonate) to 50 mL of H2O. Dissolve with a minimum volume of 1:4 HNO3, adding the acid dropwise. Dilute to 1 L to give a 1000 mg/L Ca solution.

Interferences In the air/acetylene flame, interferences such as refractory oxide depress the calcium absorbance. Addition of a releasing agent such as strontium or lanthanum is used to release calcium atoms for determination.

57


20

Ca

Calcium



Parameter


0.0096 mg/L


A bso rption Sen s itiv ity

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2C o ncen tra tion (m g /L)

Ab

so

rba

nce

• The absorption sensitivity is very dependent on the fuel flow rate and burner head position.

• The best precision is achieved with an oxidizing flame, and best sensitivity with a reducing flame.

• A wide slit width can be used for determinations in the air flame, but is not recommended for the nitrous oxide flame.


0

0.1

0.2

0.3

3 4.5 6 7.5 9Fuel Flow Rate (L/min)

Abso

rbance

Standard Solutions Add 2.497 g of dried CaCO3 (calcium carbonate) to 50 mL of H2O. Dissolve with a minimum volume of 1:4 HNO3, adding the acid dropwise. Dilute to 1 L to give a 1000 mg/L Ca solution.

Interferences In the nitrous oxide flame, ionization can be a problem, but is easily solved by adding 1% of an easily ionized alkali salt.

Abso rptio n Sen s itiv ity v s . s l it W id th

0.2

0.25

0.3

0.35

0.4

0 0 .2 0.4 0.6 0 .8

S lit W ith (n m )

Ab

so

rba

nce

Sensitivity is depressed when elements forming oxy salts are present, including beryllium, aluminum, phosphorous, titanium, and silicon. The addition of 1% lanthanum or strontium can improve sensitivity. There are no notable chemical interferences.

58


55

Cs

Cesium



Parameter

Flame: air/acetylene Flame Stoichiometry: oxidizing (lean, blue) HCL Current: 10 mA PMT Voltage: 455 V Slit Width: 0.2 nm Characteristic Concentration:

0.07 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 1.267 g of cesium chloride (CsCl) in deionized water. Dilute to 1 L to give a 1000 ppm cesium solution.

Interferences Absorption Sensitivity vs. Fuel flow Rate

0.2

0.25

0.3

0.35

0.4

0.5 1 1.5 2Fuel Flow Rate (L/min)

Abso

rban

ce

Ionization interference can be reduced by adding 0.1% of an alkali salt. Strong mineral acids can decrease the sensitivity, and so matrix matching is advised.

59


24

Cr

Chromium



Parameter


0.04 mg/L


Abso rptio n Sen s itiv ity

(3 57.87 nm , uppe r; 4 25.44 n m low er line )

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10C once ntra tion (m g /L)

Ab

so

rba

nce

Standard Solutions Dissolve 1.000 g of chromium metal wire, or potassium dichromate (K2Cr2O7), in 1:1 HCl. Heat to speed up dissolution. Dilute to 1 L to give a 1000 ppm Cr solution.

Interferences Ab sorption Se ns itiv ity vs . Fue l flow Rate

0

0 .1

0 .2

0 .3

1.5 2 2.5 3Fue l F low Ra te (L/m in)

Ab

so

rba

nce

The reducing air-acetylene flame provides the most sensitivity to Cr absorbance. However, the stability of aqueous Cr solutions is subject to the solution acidity. Different ionized states of Cr exhibit different sensitivities in the reducing air-acetylene flame. The addition of 20g/L ammonium chloride, 4g/L potassium thiocyanate, 20g/L potassium persulfate or 10g/L ammonium hydrogen fluoride, either alone or together with 2 g/L sodium sulfate may eliminate this effect. The presence of Co, Fe and Ni (particularly in perchloric acid) causes depression of Cr absorbance. Cu, Ba, Al, Mg and Ca are also reported to interfere with the Cr absorbance. The extent of the interference is strongly dependent on the flame stoichiometry. This interference can be eliminated by using oxidizing air-acetylene flame or nitrous oxide-acetylene flame with the compromise of sensitivity loss.

60


24

Cr

Chromium


Guideline Parameters - FAAS Abso rptio n Sen s itiv ity

(3 57.87 nm , uppe r; 4 25.44 n m low er line )

0

0.4

0.8

1.2


Ab

so

rba

nce

Parameter


0.068 mg/L


Standard Solutions Dissolve 1.000 g of chromium metal wire, or potassium dichromate (K2Cr2O7), in 1:1 HCl. Heat to speed up dissolution. Dilute to 1 L to give a 1000 ppm Cr solution.

Interferences Abso rpt ion Sens itivity vs . F ue l flow Rate

0

0.1

0.2

0.3

0.4

2 4 6 8

Fue l F low Ra te (L/min)

Ab

sorb

ance

The reducing air-acetylene flame provides the most sensitivity to Cr absorbance. However, the stability of aqueous Cr solutions is subject to the solution acidity. Different ionized states of Cr exhibit different sensitivities in the reducing air-acetylene flame. The addition of 20g/L ammonium chloride, 4g/L potassium thiocyanate, 20g/L potassium persulfate or 10g/L ammonium hydrogen fluoride, either alone or together with 2 g/L sodium sulfate may eliminate this effect. The presence of Co, Fe and Ni (particularly in perchloric acid) causes depression of Cr absorbance. Cu, Ba, Al, Mg and Ca are also reported to interfere with the Cr absorbance. The extent of the interference is strongly dependent on the flame stoichiometry. This interference can be eliminated by using oxidizing air-acetylene flame or nitrous oxide-acetylene flame with the compromise of sensitivity loss.

61


27

Co

Cobalt



Parameter


0.04 mg/L


Ab sorption Se ns itiv ity

(240 .7 3nm up per; 34 6.58nm low er line)

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8C o ncen tra tion (m g /L)

Ab

so

rba

nce

Standard Solutions Dissolve 1.000 g of Co metal wire in a minimum volume of 1:1 HNO3. Dilute to 1 L to give a 1000 ppm Co solution. Ab sorption Se nsitiv ity v s . Fue l flow Rate

0.2

0.3

0.4

0.5

0.5 1 1.5 2Fue l Flo w R ate (L/m in)

Ab

so

rba

nce

Interferences No major interferences to note. However, Co is found naturally with Ni, and high Ni levels (> 1.5 ppm) can depress the Co signal.

Ab sorption Se nsitiv ity v s . Lam p C urrent

0.2

0.3

0.4

0.5

0 5 10 1 5 20La m p C u rrent (m A)

Ab

so

rba

nce

62


29

Cu

Copper



Parameter


0.03 mg/L


Absorbance Sensitivity

0

0.2

0.4

0.6

0.8

1

0 2 4Concentration (mg/L)

Abso

rban

ce

6

Standard Solutions

Absorbance vs. HCL current

0

0.2

0.4

0.6

4 6 8 10 12HCL current (mA)

Abso

rption

Dissolve 1.000 g of copper metal wire in a minimum volume of 1:1 HNO3. Dilute to 1 L to give a 1000 ppm Cu solution.

Interferences No major interferences. However, when Zn is present at high concentrations, the Cu signal can be decreased. This interference can be alleviated by using a lean air/acetylene flame or a nitrous oxide/acetylene flame.

63


66

Dy

Dysprosium

Analytical lines: 421.2 nm 404.6 nm 416.8 nm


Parameter

Flame: nitrous oxide/acetyleneFlame Stoichiometry: reducing HCL Current: 15 mA PMT Voltage: 400 V Slit Width: 0.2 nm Characteristic conc.: 0.58 mg/L Working Range: 0.3-150 mg/L


0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 1.148 g of dysprosium oxide (Dy2O3) in a minimum volume of HCl. Dilute to 1 L with 1% v/v HCl to give a 1000 ppm Dy solution.

Interferences Ionization in the nitrous oxide flame can be reduced by the addition of an ionization suppressant (eg. 0.1% of an alkali salt). In the presence of H2SO4 and H3PO4, the Dy signal is enhanced. In the presence of Al, Si, HF, Dy signal is decreased (especially when Na is also present), so matrix matching is advised for such matrices. Dy is one of the lanthanide elements, which are known to display complex mutual interference patterns.

64


68

Er

Erbium



Parameter



0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 1.143 g of erbium oxide (Er2O3) in a minimum volume of HCl. Dilute to 1 L with 1% v/v HCl to give a 1000 ppm Er solution.

Interferences Ionization in the nitrous oxide flame can be reduced by the addition of an ionization suppressant (eg. 0.1% of an alkali salt). The presence of Al, Si, and HF can all decrease the Er signal when Na is present, so matrix matching is advised for such matrices. Er is one of the lanthanide elements, which are known to display complex mutual interference patterns.

65


63

Eu

Europium



Parameter

Flame: nitrous oxide/acetyleneFlame Stoichiometry: reducing HCL Current: 10 mA PMT Voltage: 350 V Slit Width: 0.2 nm Characteristic conc.: 0.4 mg/L Working Range: 5-80 mg/L


0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 1.1579 g of europium oxide (Eu2O3) in a minimum volume of HCl. Dilute to 1 L with 1% v/v HCl to give a 1000 ppm Eu solution.

Interferences Ionization in the nitrous oxide flame can be reduced by the addition of an ionization suppressant (eg. 0.1% of an alkali salt). The presence of Al, Si, and HF can all decrease the Eu signal when Na is present, so matrix matching is advised for such matrices. Eu is one of the lanthanide elements, which are known to display complex mutual interference patterns.

66


64

Gd

Gadolinium



Parameter



0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 1.1526 g of gadolinium oxide (Gd2O3) in a minimum volume of HCl. Dilute to 1 L with 1% v/v HCl to give a 1000 ppm Gd solution.

Interferences Ionization in the nitrous oxide flame can be reduced by the addition of an ionization suppressant (eg. 0.1% of an alkali salt). The presence of Al, Fe, Si, fluorides, and phosphates can all significantly decrease the Gd signal (even at moderate levels), so matrix matching is advised for such matrices. Gd is one of the lanthanide elements, which are known to display complex mutual interference patterns.

67


31

Ga

Gallium



Parameter

Flame: air/acetylene Flame Stoichiometry: oxidizing HCL Current: 5 mA PMT Voltage: 502 V Slit Width: 0.2 nm Characteristic conc.: 1.0 mg/L Working Range: 1-100 mg/L

Absorption Sensitivity (294.4nm upper; 287.4nm middle, 403.3 nm lower line)

0

0.2

0.4

0.6

0.8

1


Abso

rbance

Standard Solutions Dissolve 1.000 g of gallium metal in a minimum volume of aqua regia (1:3 HNO3:HCl). Heat to aid dissolution. Cool and dilute to 1 L with 1% v/v HCl to give a 1000 ppm Ga solution.

Interferences Absorption Sensitivity vs. Fuel flow Rate

0

0.1

0.2

0.3

0.5 1 1.5 2 2.5 3Fuel Flow Rate (L/min)

Abso

rban

ce

Ionization interference can occur in the nitrous oxide acetylene flame, but it is eliminated by the addition of an ionization suppressant (an alkali salt). The sensitivity in an air/acetylene flame is roughly half that of the nitrous oxide/acetylene flame at the 287.4 nm line.

68


32

Ge

Germanium



Parameter


1.26 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

• the 265.2 nm line is a doublet (265.1/265.2 nm)

Standard Solutions Dissolve 1.000 g in 5 mL of conc. HF in a Teflon vessel. Slowly add conc. HNO3 dropwise until the dissolution is complete. Dilute to 1 L with H2O to give a 1000 ppm Ge solution. Store the solutions in polyethylene bottles, not glass.

WARNING: HF (hydrofluoric acid) is a very toxic acid and should be handled with extreme care. Personal protective equipment is essential.

Interferences No interferences to report.

69


79

Au

Gold



Parameter


0.085 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

• The 242.9nm line is more sensitive, but the 267.6nm line can be more precise because it is more intense.

Standard Solutions Absorption Sensitivity vs. Fuel flow Rate

0.1

0.15

0.2

0.25


Abso

rban

ce

Dissolve 1.000 g of Au metal foil in a minimum volume of aqua regia. Dilute to 1 L to give a 1000 ppm Au solution. Store standard solutions in amber bottles. Make working solutions with 10% HCl.

Interferences Large quantities of mineral acids and elements such as iron, copper, calcium, cobalt, tin and other noble metals can depress signal. Adding 1% uranium, as a releasing agent, or dilute Cu-Cd buffers, can alleviate these effects.

Absorption Sensitivity vs. Lamp Current

0

0.1

0.2

0.3

0 5 10 15 20 25Lamp Current (mA)

Abso

rban

ce

Nitrous oxide-acetylene flame can reduce the interferences but the sensitivity is also reduced.

70


72

Hf

Hafnium



Parameter

Flame: nitrous oxide/acetyleneFlame Stoichiometry: reducing HCL Current: 10 mA PMT Voltage: 400 V Slit Width: 0.2 nm Characteristic conc.: 11 mg/L Working Range: 20-2500 mg/L


0

0.2

0.4

0.6

0.8

1

0 500 1000 1500 2000 2500Concentration (mg/L)

Abso

rban

ce

Standard Solutions Dissolve 1.000 g of hafnium metal in a Teflon vessel by slowly adding 5 mL of HF dropwise. Slowly add 10 mL of HNO3 dropwise, allowing the reaction to diminish between additions. Cool and dilute to 1 L to give a 1000 ppm Hf solution. Store the solutions in polyethylene bottles.

WARNING: HF (hydrofluoric acid) is a very toxic acid and should be handled with extreme care. Personal protection equipment is essential.

Interferences At higher hafnium concentrations, oxides can form. This can create calibration curve rollover. To eliminate this effect, add 0.2% Al and 1% HF to all solutions. In general, the Hf signal is decreased by alkali and alkaline earth metals, and H2SO4. Many transition metals also cause interferences. The use of an oxidizing nitrous oxide/acetylene flame can often reduce these interferences.

71


67

Ho

Holmium



Parameter



0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 1.146 g of holmium oxide (Ho2O3) in a minimum volume of HCl. Dilute to 1 L with 1% v/v HCl to give a 1000 ppm Ho solution.

Interferences Ionization in the nitrous oxide flame can be reduced by the addition of an ionization suppressant (eg. 0.1% of an alkali salt). The presence of Al, Si, and HF can all decrease the Ho signal, so matrix matching is advised for such matrices.

72


49

In

Indium



Parameter


0.2 mg/L


Abs orptio n Sen sitiv ity

0

0.2

0.4

0.6

0.8

1

0 10 2 0 3 0 40C onc entra tio n (m g/L)

Ab

so

rba

nce

Standard Solutions Dissolve 1.000 g of indium metal wire in a minimum volume of 1:1 HCl. Gentle heating and adding ~1mL of HNO3 will aid dissolution. Dilute to 1 L with 1% (v/v) HCl to give a 1000 ppm indium solution.


0.2

0.3

0.4

0.5


Abso

rban

ce

Interferences

When large excesses of Cu, Zn, Sn, Al, Fe, Mg, Si or phosphate are present, the sensitivity is decreased. Matrix matching of samples and standards is advised. Ionization occurs in the nitrous oxide/acetylene flame.

73


77

Ir

Iridium



Parameter


0.8 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rbance

Standard Solutions Dissolve 2.294 g of (NH4)2IrCl6 (ammonium hexachloroiridate) in a minimum volume of 1% v/v HCl. Dilute to 1 L with 1% HCl to give a 1000 ppm Ir solution.

Interferences There are many interferences to note. Some metals (Al, Cu, K, La, Na, Pb, and Pt) increase the signal, and other metals (Fe, Ni, Pd, Sn, and Ti) decrease the signal. The degree of interference depends largely on the concentration ratio of Ir to interfering species. Therefore, matrix matching of all solutions is important. Many interferences are eliminated in the nitrous oxide/acetylene flame, but sensitivity is generally decreased by 50%.

74


26

Fe

Iron



Parameter


0.045 mg/L


A bso rp tion S ens itiv ity

(D ou ble t 24 8.33 nm , up per line;

3 71.99 nm , low er line )

0

0 .2

0 .4

0 .6

0 .8

1


Ab

so

rba

nce

Standard Solutions Dissolve 1.000 g of iron metal strip in 20mL of 1:1 HCl. Dilute to 1 L to give a 1000 ppm Fe solution.

Interferences Abs orptio n Sen s itiv ity v s . Fu e l f low

Rate

0 .1

0 .2

0 .3

0 .4

0 .5

0.5 1 1.5 2Fue l Flo w R ate (L/m in)

Ab

so

rba

nce

Silicon, aluminum, manganese, and sulfates can all depress the signal, but the effect gets weaker at higher measurement heights in the flame. Citric acid depresses the absorption signal, and the effect can be reduced by adding phosphoric acid to the samples. Two oxidation states of Fe can occur in a reducing flame.

A bsorp tion S ens itiv ity vs . Lam p C urren t

0.2

0.3

0.4

0.5

0 5 1 0 15 2 0 25La m p C u rre nt (m A )

Ab

so

rba

nce

75


57

La

Lanthanum



Parameter


60 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rbance

Standard Solutions Dissolve 11.730 g of La2O3 (lanthanum oxide) in a minimum volume of 1:1 HNO3. Dilute to 1 L to give a 10,000 ppm La solution.

Interferences Ionization interference occurs in the nitrous oxide/acetylene flame. Add an ionization suppressant (K, Cs, or another alkali) to reduce this effect. Al, Fe, Si, fluorides, and phosphates all depress the La signal. Matrix matching of all solutions is recommended.

76


82

Pb

Lead



Parameter


0.08 mg/L


Absorption Sensitivity(upper line 217 nm, lower line 283.30nm)

0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 1.598 g of lead nitrate (Pb(NO3)2) in 1% (v/v) HNO3. Dilute to 1 L to give a 1000 ppm lead solution. OR Dissolve 1.000 g of lead metal wire in 1:1 HNO3. Dilute to 1 L with deionized water to give a 1000 ppm lead solution.

WARNING: This element is toxic and should be handled with extreme care. Personal protective equipment is essential.


0

0.05

0.1

0.15

0.2

1 1.5 2 2.5Fuel Flow Rate (L/min)

Abso

rban

ce

Interferences There are no cationic interferences, but several anionic ones. At high concentrations (relative to lead), phosphate, carbonate, iodide, fluoride, and acetate anions can all depress the lead signal. These interferences can be alleviated by the addition of 0.1 M EDTA.


0

0.05

0.1

0.15

0.2

0 5 10 15 20Lamp Current (mA)

Abso

rban

e

For multielement lamps with copper, the copper 216.5nm line may interfere with the 217nm lead line. The 283.3nm lead line should be used instead.

77


3

Li

Lithium



Parameter


0.01 mg/L



0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5 3Concentration (mg/L)

Abso

rban

ce

Standard Solutions Dissolve 5.324 g of lithium carbonate (Li2CO3) in a minimum volume of 1:1 HCl. Dilute to 1 L to give a 1000 ppm Li solution.


0.1

0.2

0.3

0.4


Abso

rban

ce

Interferences Ionization is a consideration, but can be compensated for by adding K. For geological samples, interferences are from the presence of high concentrations of aluminum, phosphate, fluoride and perchlorate. Nitrous oxide/acetylene flame can remove the above interferences.


0.1

0.2

0.3

0.4


Abso

rban

ce

78


71

Lu

Lutetium



Parameter



0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 1.137 g of lutetium oxide (Lu2O3) in a minimum volume of 1:1 HNO3. Dilute to 100 mL to give a 10,000 ppm Lu solution.

Interferences Ionization in the nitrous oxide flame can be reduced by the addition of an ionization suppressant (eg. 0.1% of an alkali salt). The presence of Al, Fe, Si, fluorides, and phosphates can all decrease the Lu signal, so matrix matching is advised for such matrices. Lu is one of the lanthanide elements, which are known to display complex mutual interference patterns.

79


12

Mg

Magnesium



Parameter


0.0024 mg/L


A bsorp tion S ens itiv ity

(28 5.21 nm , up pe r; 202 .5 8 nm , low er)

0

0.2

0.4

0.6

0.8

1

0 0 .2 0.4 0 .6 0.8C o ncen tra tion (m g/L)

Ab

so

rba

nce

Standard Solutions Dissolve 1.000 g of magnesium metal strip slowly in a minimum volume of 1:1 HCl. Dilute to 1 L to give a 1000 ppm Mg solution. Ab sorptio n Se ns itiv ity v s . Fu e l f low R ate

0

0.1

0.2

0.3

0.5 1 1.5 2 2.5Fue l F low Ra te (L/m in)

Ab

so

rba

nce

Interferences Interferences from higher concentrations of Si, Al, Ti, and P can be eliminated by adding lanthanum chloride. Ionization occurs in the nitrous oxide/acetylene flame, so ionization suppressants must be added to solutions.

80


25

Mn

Manganese



Parameter


0.021 mg/L


Absorption Sensitivity(Doublet 279.8/280.1 nm, upper line; 279.48nm, lower line)

0

0.3

0.6

0.9

1.2


Abso

rban

ce

• the S/N ratio can be improved by using a wide slit width and measuring absorption with the triplet 279.5/279.8/280.1 nm. However, the characteristic concentration is 0.1 mg/L and calibration linearity is compromised.

Standard Solutions Absorption Sensitivity vs. Fuel flow Rate

0.1

0.2

0.3

0.4

0.5


Abso

rban

ce

Dissolve 1.000 g of manganese metal wire in a minimum volume of 1:1 HNO3. Dilute to 1 L to give a 1000 ppm Mn solution.

Interferences Signal suppression was observed in the presence of silicon, phosphorus, borate, tungstate, dichromate, silicate, cyanide, and rhenium. The use of a nitrous oxide/acetylene flame can reduce these interferences. However, sensitivity is sacrificed. Some complexing agents (eg. EDTA, NTA) can increase the signal in an air/acetylene flame. Absorption Sensitivity vs. Lamp Current

0.2

0.3

0.4

0.5

0 5 10 15 20 25Lamp Current (mA)

Abso

rban

ce

81


80

Hg

Mercury

Analytical lines: 253.7 nm


Parameter


3 mg/L



0

0.2

0.4

0.6

0.8

1


Abs

orba

nce

Standard Solutions Dissolve 1.080 g of mercury (II) oxide (HgO) in a minimum volume of 1:1 HCl. Dilute to 1 L with deionized water to give a 1000 ppm mercury solution. OR Absorption Sensitivity vs. Fuel flow Rate

0

0.05

0.1

0.15

0.2


Abso

rban

ce

Dissolve 1.354 g of mercuric (II) chloride (HgCl2) in 10mL of HNO3. Dilute to 1 L with deionized water to give a 1000 ppm mercury solution.

WARNING: Mercury is very toxic and should be handled with extreme care. Personal protective equipment is essential.

Interferences Mercury (I) is more sensitive than mercury (II). Mercury (II) can be reduced to mercury (I) by ascorbic acid, stannous chloride, and other reductants, thus falsely increasing the signal. High cobalt levels will interfere with the 253.7nm mercury line. It is advisable to check and correct for non-specific absorption.

82


42

Mo

Molybdenum


A bso rption Sen sitiv ity

0

0.2

0.4

0.6

0.8

1

0 25 50 75 1 00 125C o ncen tra tion (m g/L)

Ab

so

rba

nce


Parameter


0.64 mg/L


Standard Solutions Dissolve 1.000 g of molybdenum metal strip into hot conc. HNO3. Cool and dilute to 1 L to give a 1000 ppm Mo solution. OR Dissolve 1.840 g of (NH4)6Mo7O24 4H2O (ammonium heptamolybdate tetrahydrate) in 1% v/v NH4OH (ammonium hydroxide) and make up to 1 L with the NH4OH to give a 1000 ppm Mo solution.

Interferences The signal can be depressed in the presence of Ca, Fe, Sr, and sulfate. The effects can be reduced by the addition of 1-3 g/L aluminum. When Mo is determined in an air/acetylene flame, the sensitivities are 3-10 times poorer than those obtained with a nitrous oxide/acetylene flame. As well, there are far fewer interferences in the nitrous oxide flame.

83


60

Nd

Neodymium



Parameter



0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 1.167 g of neodymium oxide (Nd2O3) in a minimum volume of HNO3. Dilute to 100 mL with 1% v/v HNO3 to give a 10,000 ppm Nd solution.

Interferences Ionization in the nitrous oxide flame can be reduced by the addition of an ionization suppressant (eg. 0.1% of an alkali salt). The presence of Al, Fe, Si, and HF can all decrease the Nd signal, so matrix matching is advised for such matrices. Nd is one of the lanthanide elements, which are known to display complex mutual interference patterns. The presence of Pr (another lanthanide element) can cause interference with the overlap of its 492.5 nm line. Neodymium, Nd

84


28

Ni

Nickel



Parameter


0.042 mg/L


Abso rptio n Sen sitiv ity

0

0 .2

0 .4

0 .6

0 .8

1

0 3 6 9 1 2

C once ntra tion (m g /L)

Ab

so

rba

nce

Standard Solutions Dissolve 1.000 g of nickel metal wire in 1:1 HNO3. Dilute to 1 L in 1% v/v HNO3 to give a 1000 ppm Ni solution.


0.2

0.3

0.4

0.5


Abso

rban

ce

Interferences At the 232.0 nm absorption wavelength, it is common for non-specific absorption to occur. This is especially a problem when non-dissolved species are present. To alleviate this interference, a background correction method must be employed. Non-specific absorption is not a problem at the 341.5 nm wavelength. Fe or Cr present at high levels can increase the Ni signal. The use of a nitrous oxide/acetylene flame will remove most interferences, but signal sensitivity will be sacrificed. A bso rp tion S ens itiv ity vs . Lam p C urre nt

0.2

0.3

0.4

0.5

0 5 1 0 1 5 20La m p C u rre nt (m A )

Ab

so

rba

nce

85


41

Nb

Niobium



Parameter


23 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 5.000 g of niobium metal in a Teflon vessel by slowly adding 5 mL of conc. HF dropwise. Add 10 mL of conc. HNO3 dropwise to complete the dissolution. Dilute to 1 L to give a 5,000 ppm Nb solution. Store solutions in polyethylene bottles.


Interferences At higher concentrations of niobium, oxide formation can occur. This results in calibration curve roll-over. This effect can be countered by adding 0.2% Al and 1% HF. Ionization interference is common, but can be reduced by the addition of an ionization suppressant (an alkali salt such as K or Cs).

86


76

Os

Osmium



Parameter


2.2 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rbance

Standard Solutions Obtain a certified stock standard solution of 1000 ppm osmium from a chemicals supplier.

OR Dissolve 1.340 g of OsO4 (osmium tetroxide) in 100 mL of H2O. Add a NaOH (sodium hydroxide) pellet to aid the dissolution. Dilute to 1 L to give a 1000 ppm Os solution. Os solutions are unstable and fresh solutions should be prepared frequently. Store solutions in glass bottles. All solutions made from this stock must be made with 1% v/v H2SO4.

WARNING: OsO4 is very toxic and volatile. Avoid inhaling the vapors. Handle this substance with extreme care. Personal protective equipment is essential.

Interferences The nitrous oxide/acetylene flame is about 5 times more sensitive than the air/acetylene flame.

87


46

Pd

Palladium



Parameter


0.05 mg/L


Absorption Sensitivity(244.79 upper; 247.64 lower line)

0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 1.000 g of palladium metal wire in a minimum volume of aqua regia. Evaporate to dryness. Dissolve in 5mL of conc. HCl and 25mL warm water. Dilute to 1 L to give a 1000 ppm palladium solution.


0.3

0.35

0.4

0.45

0.5


Abso

rban

ce

OR Dissolve 2.672 g of ammonium chloropalladite ((NH4)2PdCl4) in deionized water. Dilute to 1 L to give a 1000 ppm palladium solution.

Interferences A bsorption S ensitiv ity vs . Lam p C urren t

0.2

0.3

0.4

0.5

0 5 1 0 1 5 20Lam p C urre nt (m A )

Ab

so

rba

nce

The signal is decreased by cobalt, nickel, aluminum, and HF. The interference can be improved by adding lanthanum or 0.01 M EDTA, and also by measuring higher in the flame. The nitrous oxide/acetylene flame removes many interferences, but sensitivity is greatly sacrificed.

88


15

P

Phosphorous



Parameter


92 mg/L

Working Range: 400-30000 mg/L • the analytical lines are a doublet: 213.6/214.9 nm.


0

0.4

0.8

1.2


Abso

rban

ce

Standard Solutions Dissolve 37.138 g of dry NH4H2PO4 (ammonium dihydrogen orthophosphate) in H2O. Dilute to 1 L to give a 10,000 ppm P solution.

Interferences There is little mention of interferences in the literature. However, matrix matching of solutions is advised as a precaution.

89


78

Pt

Platinum



Parameter


0.6 mg/L



0

0.2

0.4

0.6

0.8

1


Abs

orba

nce

Standard Solutions Dissolve 2.275 g of ammonium chloroplatinate ((NH4)2PtCl6) in deionized water. Dilute to 1 L with more water to give a 1000 ppm platinum solution.


0.2

0.25

0.3

0.35

0.4


Abso

rban

ce

Interferences High levels of most noble metals and acids can decrease the platinum signal. These can be compensated for by adding 0.2% lanthanum in 1.0% HCl or by making solutions 2% in copper. Most interferences are eliminated in the nitrous oxide/acetylene flame, but the sensitivity is sacrificed. It is advisable to matrix-match standards with the samples. Absorption Sensitivity vs. Lamp Current

0.3

0.35

0.4


Abso

rban

ce

90


19

K

Potassium



Parameter


0.0066 mg/L



(upp er line 76 6.49 nm , lo we r line 769 .9 0

nm )

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2C on centra tion (m g/L)

Ab

so

rba

nce

Standard Solutions Dissolve 1.907 g of dry potassium chloride (KCl) in H2O. Dilute to 1 L to give a 1000 ppm K solution.

Absorption Sensitivity vs. Fuel flow Rate(0.5 mg/L)

0.1

0.2

0.3

0.4

0.9 1.2 1.5 1.8Fuel Flow Rate (L/min)

Abso

rbance

Interferences K can be partially ionized in the flame. The addition of another easily ionizable element, such as La or Cs salts, can suppress this effect. High levels of mineral acids can depress the signal.

91


Pr

Praseodymium


59


Parameter

Flame: nitrous oxide/acetyleneFlame Stoichiometry: reducing HCL Current: 10 mA PMT Voltage: 450 V Slit Width: 0.2 nm Characteristic conc.: 33 mg/L Working Range: 100-5000 mg/L


0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 1.170 g of praseodymium oxide (Pr2O3) in a minimum volume of HCl. Dilute to 100 mL with 1% v/v HCl to give a 10,000 ppm Pr solution.

Interferences Ionization in the nitrous oxide flame can be reduced by the addition of an ionization suppressant (eg. 0.1% of an alkali salt). An excess of Si decreases the Pr signal. Pr is one of the lanthanide elements, which are known to display complex mutual interference patterns.

92


75

Re

Rhenium



Parameter


14.7 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rbance

Standard Solutions Dissolve 1.554 g of KReO4 (potassium perrhenate) in H2O. Dilute to 1 L with 1% v/v H2SO4 to give a 1000 ppm Re solution.

Interferences The signal is depressed by the presence of Al, Ba, Ca, Fe, K, Mg, Mn, Mo, Pb, and most transition group metals. The use of a more oxidizing flame can often alleviate these interferences somewhat. Matrix matching of all solutions is recommended.

93


45

Rh

Rhodium



Parameter


0.11 mg/L


Absorption Sensitivity(343.49nm, upper; 339.69 nm lower line)

0

0.2

0.4

0.6

0.8

1


Abs

orba

nce

Standard Solutions Dissolve 4.120 g of ammonium hexachlororhodate trihydrate ((NH4)3RhCl6 3H2O) in a minimum volume of 10% (v/v) HCl. Dilute to 1 L with 10% (v/v) HCl to give a 1000 ppm rhodium solution.


0.4

0.45

0.5


Abso

rban

ce

Working standard solutions should also be made with 10% HCl.

Interferences Adding 1.0% Na2SO4 relieves some chemical interferences, and also improves linearity and sensitivity. Most elements interfere unpredictably. Phosphoric and sulfuric acids decrease the sensitivity, while alkali metal sulfates increase sensitivity. The nitrous oxide/acetylene flame removes many interferences, but sensitivity is sacrificed.

94


37

Rb

Rubidium



Parameter


0.015 mg/L


Absorption Sensitivity(780.0nm, upper line; 794.8 nm lower line)

0

0.2

0.4

0.6

0.8

1


Abs

orba

nce

Standard Solutions Dissolve 1.415 g of rubidium chloride (RbCl) in deionized water. Dilute to 1 L to give a 1000 ppm rubidium solution. Absorption Sensitivity vs. Fuel flow Rate

0.5

0.55

0.6

0.65


Abso

rban

ce

Interferences Rubidium is relatively easily ionized, but this can be controlled by adding 0.1% alkali salt. Aluminum and mineral acids can depress the signal in the lower flame region, and so matrix matching of the standards to the samples is advised.

95


44

Ru

Ruthenium



Parameter


0.57 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 2.052 g of ruthenium chloride (RuCl3 3H2O) in 25% (v/v) HCl. Dilute to 1 L with 25% (v/v) HCl to give a 1000 ppm ruthenium solution.

Interferences Ruthenium sensitivity is decreased by molybdenum, but increased by platinum, rhodium, lanthanum, and HCl. In general, however, most elements interfere unpredictably. The nitrous oxide/acetylene flame removes most interferences, and the addition of 0.5% lanthanum improves sensitivity.

96


62

Sm

Samarium



Parameter



0

0.2

0.4

0.6

0.8

1


Abso

rbance

Standard Solutions Dissolve 1.159 g of samarium oxide (Sm2O3) in a minimum volume of HCl. Dilute to 100 mL with 10% v/v HCl to give a 10,000 ppm Sm solution.

Interferences Ionization in the nitrous oxide flame can be reduced by the addition of an ionization suppressant (eg. 0.1% of an alkali salt). Sm is one of the Lanthanide elements, which are known to display complex mutual interference patterns.

97


21

Sc

Scandium



Parameter


0.40 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 1.534 g of Sc2O3 (scandium oxide) in a minimum volume of 1:1 HCl. Dilute to 1 L to give a 1000 ppm Sc solution.

Interferences Ionization in the nitrous oxide/acetylene flame can be controlled by the addition of an alkali salt to all solutions. Significant signal depression occurs when fluoride, sulfide, or sulfate anions are present. Many cations are known to increase or decrease the signal when present at high levels. To compensate for these interferences, matrix matching of all solutions is advised.

98


34

Se

Selenium



Parameter


0.17 mg/L


A bsorp tion S ens itiv ity

0

0.2

0.4

0.6

0.8

1

0 10 2 0 3 0 40C onc entra tio n (m g/L)

Ab

so

rba

nce

Standard Solutions Dissolve 1.000 g of selenium metal pellets in a minimum volume of 1:1 HNO3. Heat gently to help the dissolution. Cool the solution and dilute to 1 L to give a 1000 ppm Se solution.


0.1

0.15

0.2

0.25

0.3

1 1.2 1.4 1.6 1.8Fuel Flow Rate (L/min)

Abso

rban

ce

WARNING: Selenium is a toxic element and should be handled with extreme care. Personal protective equipment is essential.

Interferences The analytical line of 196.03 nm is at the start of UV range, where the flame gases can absorb and scatter part of the radiation. Therefore, use of background correction is recommended. The absorption by the flame is reduced when a nitrous oxide/acetylene flame is used, but sensitivity is also reduced.

99


14

Si

Silicon



Parameter

Flame: nitrous oxide/acetylene Flame Stoichiometry: reducing HCL Current: 5 mA PMT Voltage: 497 V Slit Width: 0.2 nm Characteristic conc.: 1.2 mg/L Working Range: 3-400 mg/L

Absorption Sensitivity(upper line 251.6nm, lower line 221.1nm)

0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Fuse 2.139 g of silicon dioxide with 8 g of NaOH (sodium hydroxide) in a zirconium crucible. Cool the melt and dissolve it with 100 mL of 1:3 HCl. Dilute to 1 L to give a 1000 ppm Si solution.


0

0.1

0.2

0.3

5 6 7 8 9 10

Fuel Flow Rate (L/min)

Abso

rban

ce

OR Fuse 2.139 g of silicon dioxide with 20 g of Na2CO3 (sodium carbonate) in a platinum crucible. Cool the melt and dissolve it with H2O. Dilute to 1 L to give a 1000 ppm Si solution.

Interferences In acidic solutions, silicon can precipitate out and cause lower signals. To keep Si in solution, add 1% HF. Large excess of alkali and aluminum causes signal enhancement.

100


47

Ag

Silver



Parameter


0.016 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

• Ag sensitivity can be poor due to the low dissociation of Ag in solutions.

Standard Solutions Dissolve 1.000 g of Ag metal strip in 20 mL of 1:1 nitric acid. Dilute to 1 L to give a 1000 ppm Ag solution.


0

0.1

0.2

0.3


Abso

rban

ce

Silver is sensitive to light. Store stock solutions in amber bottles and in closed cupboards.

Interferences Acetic acid increases absorption. Mineral acids and aluminum decrease absorption. Silver is precipitated by bromide, chloride, chromate, iodide, iodate, permanganate, and tungstate. Absorption Sensitivity vs. Fuel flow Rate

0

0.05

0.1

0.15

0.2


Abso

rban

ce

101


11

Na

Sodium

Analytical lines: 589.0 nm 589.6 nm 330.2 nm 330.3 nm


Parameter


0.0032 mg/L



(589 .0 0nm up per; 58 9.59nm low er line )

0

0.2

0.4

0.6

0.8

1

0 0 .2 0.4 0.6 0.8 1C once ntra tio n (m g/L)

Ab

so

rba

nce

• the 589.0/589.6 nm doublet can be used to increase the S/N ratio, but calibration linearity is sacrificed.

• the 589.0 nm line is more sensitive than the 589.6 nm line.


0.25

0.3

0.35

0.4


Abso

rbance

• the 330.2/330.3 nm lines are also a doublet.

Standard Solutions Dissolve 2.542 g of dry NaCl salt (sodium chloride) in H2O. Dilute to 1 L to give a 1000 ppm Na solution.

Interferences Na is easily ionized in the flame, so an ionization suppressant must be added to eliminate this interference (Cs, K, or some other alkali salt).

Absorption Sensitivity vs. Addition of Ionization Suppressant

0.2

0.3

0.4

0.5

0.6

before afterAddition of 1 g/L Cs

Abso

rban

ce

High levels of mineral acids can decrease the sensitivity.

102


38

Sr

Strontium



Parameter


0.018 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rbance

• Data collected with potassium chloride added (final concentration of 2g/L) to suppress ionization.

Standard Solutions Dissolve 2.415 g of Sr(NO3)2 (strontium nitrate) in 100 mL of H2O and 10 mL of conc. HCl. Dilute to 1 L to give a 1000 ppm Sr solution. OR Dissolve 1.685 g of SrCO3 (strontium carbonate) in 10 mL of 1:1 HNO3. Dilute to 1 L to give a 1000 ppm Sr solution.

Interferences The air/acetylene flame is not as sensitive as the nitrous oxide/acetylene flame, and more chemical interferences are known in the air flame. In the air flame, Al, Si, Ti, Zr, phosphate, and sulfate all decrease the signal. To alleviate these effects, add 1% La or the complexing agent EDTA to all solutions. Strontium undergoes ionization in both the nitrous oxide and air flames. To reduce this interference, add an ionization suppressant (eg. K, Cs, or other alkali salt) to all solutions.

103


73

Ta

Tantalum



Parameter


12 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rbance

Standard Solutions Dissolve 1.000 g of tantalum metal strip in a Teflon vessel by carefully adding 5 mL of conc. HF dropwise. Dissolve the remaining metal by slowly adding conc. HNO3 dropwise. Dilute to 200 mL to give a 5,000 ppm Ta solution. Store all solutions in polyethylene bottles.

WARNING: HF (hydrofluoric acid) is a toxic acid and should be handled with extreme care. Personal protective equipment is essential.

Interferences Oxide formation in the flame is common and results in calibration curve roll-over at higher concentrations. To alleviate this problem and improve calibration curve linearity, add 0.2% Al and 1% HF to all solutions. The signal is interfered with in varying ways when alkali metals, sulfates, phosphates, and mineral acids are present in various combinations. Matrix matching of all solutions is recommended.

104


52

Te

Tellurium



Parameter


0.23 mg/L


Abs orptio n Sen sitiv ity

0

0.2

0.4

0.6

0.8

1

0 2 0 40 60C onc entra tio n (m g/L)

Ab

so

rba

nce

Standard Solutions Dissolve 1.000 g of tellurium metal granules in a minimum volume of 1:1:1 HNO3:HCl:water. Heat to aid dissolution, but do not boil. Dilute to 1 L with 1% (v/v) HCl to give a 1000 ppm tellurium solution. Absorption Sensitivity vs. Fuel flow Rate

0.3

0.35

0.4

0.45


Abso

rban

ce

Interferences Interference occurs in the presence of high levels of Ca, Cu, Si, Na, Zn, and Zr. Matrix matching of standards is advised. At low tellurium concentrations, acids and dissolved carbon dioxide can cause non-specific absorption, so the use of background correction is prudent.

105


81

Tl

Thallium



Parameter

Flame: air/acetylene Flame Stoichiometry: oxidizing HCL Current: 5 mA PMT Voltage: 484 V Slit Width: 0.2 nm Characteristic conc.: 0.13 mg/L Working Range: 0.1-20 mg/L

A bsorp tion S ensitiv ity

(27 6.8nm up per; 377 .6nm lo we r l in e)

0

0.2

0.4

0.6

0.8

1

0 5 1 0 15 20 25 30C once ntra tion (m g /L)

Ab

so

rba

nce

Standard Solutions Dissolve 1.303 g of thallous nitrate (TlNO3) in 20mL of 1% (v/v) HNO3. Dilute to 1 L with deionized water to give a 1000 ppm thallium solution.


0.2

0.25

0.3

0.35

0.4


Abso

rban

ce

WARNING: This element is toxic and should be handled with extreme care. Personal protective equipment is essential.

Interferences Many significant interferences exist. Matrix matching of standards is advised for samples with high levels of interfering elements. Ionization occurs in the nitrous oxide/acetylene flame.

106


50

Sn

Tin



Parameter


1.5 mg/L


A bso rp tion S ens itiv ity

(2 24.61 n m , up pe r; 286 .3 3 nm lo we r l in e)

0

0.2

0.4

0.6

0.8

0 50 1 00 150 2 00 250C on centra tion (m g/L)

Ab

so

rba

nce

Standard Solutions Dissolve 1.000 g of tin metal granules in 100 mL of conc. HCl. If necessary, heat gently to aid dissolution. Cool and dilute to 1 L to give a 1000 ppm Sn solution.

Ab sorptio n Se ns itiv ity v s . Fu e l f low R ate

0

0.1

0.2

0.3

0.4

4 5 6 7 8Fue l Flow R ate (L/m in)

Ab

so

rba

nce

Solutions made from this bulk standard should all be made with 10 % v/v HCl.

Interferences While the nitrous oxide/acetylene flame is not quite as sensitive as the air/acetylene flame, it is preferred because it has significantly less interferences, particularly when complex matrices are involved.

A bsorp tion S ens itiv ity vs . S l it W id th

0

0.1

0.2

0.3

0.4

0 0 .2 0.4 0 .6 0.8S lit W idth (nm )

Ab

so

rba

nce

107


22

Ti

Titanium



Parameter



0

0.2

0.4

0.6

0.8

1


Abso

rbance• Data collected with potassium chloride added

(final concentration of 2g/L) to suppress ionization.

Standard Solutions Dissolve 1.000 g of titanium metal in 10 mL of HF and 10 mL H2O by slowly adding (dropwise) 20 mL of HNO3. After each addition, allow the reaction to diminish before the next addition. Dilute to 1 L to give a 1000 ppm Ti solution. OR Dissolve 1.000 g of titanium metal in 100 mL of 1:1 HCl. Cool and dilute to 1 L to give a 1000 ppm Ti solution. Solutions made from this bulk standard should all be made with 10% v/v HCl.

Interferences Ionization interferences can be relieved by adding an alkali salt as an ionization suppressant. Most metals, as well as fluoride, chloride, and ammonium, will increase the titanium signal.

108


74

W

Tungsten



Parameter

Flame: nitrous oxide/acetylene Flame Stoichiometry: reducing HCL Current: 13 mA PMT Voltage: 450 V Slit Width: 0.2 nm Characteristic Concentration:

7.9 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rbance

• the 400.9 nm line is not the most sensitive line, but it affords a good S/N ratio.

Standard Solutions Dissolve 1.000 g of tungsten metal in 20 mL HNO3 and 10 mL HF in a PTFE vessel. Dilute to 1 L to give a 1000 ppm W solution. Store all solutions in polyethylene bottles.


OR Dissolve 8.975 g of Na2WO4.2H2O (sodium tungstate) in 20 mL H2O. Mix with 10 mL of 10% w/v NaOH. Dilute to 1 L to give a 5,000 ppm W solution. Store all solutions in polyethylene bottles.

Interferences High levels of some mineral acids combined with the presence of some elements (eg. Co, Cu, Fe, and K) can cause interference. Matrix matching of all solutions is recommended.

109


92

U

Uranium



Parameter

Flame: nitrous oxide/acetylene Flame Stoichiometry: reducing HCL Current: 10 mA PMT Voltage: 450 V Slit Width: 0.2 nm Characteristic conc.: 180 mg/L Working Range: 400 - 30000 mg/L


0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 21.10 g of uranyl nitrate hexahydrate (UO2(NO3)2 6H2O) in water. Dilute to 1 L to give a 10,000 ppm U solution.

WARNING: U is a radioactive element, and should be handled and stored with appropriate care. Personal protective equipment is also advised.

Interferences Ionization interferences can be countered by adding 0.1 % of an ionization suppressant (eg. alkali salt). In a complex matrix with excesses of other elements (eg. Al, Co, Fe, Ni, Pb), the U signal can be increased. Therefore, matrix matching is required for such matrices.

110


23

V

Vanadium



Parameter


0.44 mg/L


Absorption Sensitivity (318.4 nm upper; 306.6 nm lower line)

0

0.2

0.4

0.6

0.8

1


Abso

rbance• Data collected with potassium chloride added

(final concentration of 2g/L) to suppress ionization.

Standard Solutions Dissolve 1.000 g of Vanadium metal granules in a minimum volume of HNO3. Dilute to 1 L with 1% v/v HNO3 to give a 1000 ppm V solution.

Interferences Ionization interferences can be controlled by the addition of an ionization suppressant (alkali salt) to all solutions. High levels of Al, Fe, Ti, ammonium and H2SO4 can increase the signal. Some interference can be reduced by adding an excess of Al. Matrix matching of all solutions is advised.

111


70

Yb

Ytterbium



0

0.2

0.4

0.6

0.8

1


Abso

rban

ce


Parameter

Flame: nitrous oxide/acetyleneFlame Stoichiometry: reducing

(rich, red cone 1-2 cm)HCL Current: 5 mA PMT Voltage: 350 V Slit Width: 0.2 nm Characteristic conc.: 0.07 mg/L Working Range: 0.04-15 mg/L

Standard Solutions Dissolve 1.139 g of ytterbium oxide (Yb2O3) in a minimum volume of HCl. Dilute to 1 L with 1% v/v HCl to give a 1000 ppm Yb solution.

Absorbance vs. HCL current

0

0.2

0.4

0.6

4 6 8 10 12HCL current (mA)

Abso

rption

Interferences Ionization in the nitrous oxide flame can be reduced by the addition of an ionization suppressant (eg. 0.1% of an alkali salt). The presence of Al, Fe, Si, and HF can all decrease the Yb signal, so matrix matching is advised for such matrices. Yb is one of the Lanthanide elements, which are known to display complex mutual interference patterns.

112


39

Y

Yttrium



Parameter

Flame: nitrous oxide/acetylene Flame Stoichiometry: reducing HCL Current: 10 mA PMT Voltage: 400 V Slit Width: 0.2 nm Characteristic Concentration:

5.5 mg/L



0

0.2

0.4

0.6

0.8

1


Abso

rbance

Standard Solutions Dissolve 1.270 g of Y2O3 (yttrium oxide) in minimum volume of HCl. Dilute to 1 L with 1% v/v HCl to give a 1000 ppm Y solution.

Interferences To reduce ionization interferences, add an ionization suppressant (alkali salt) to all solutions. The signal is depressed by the presence of Al, K and some mineral acids. Matrix matching of all solutions is recommended.

113


30

Zn

Zinc



Parameter


0.008 mg/L


Abso rptio n Sen s itiv ity

0

0.2

0.4

0.6

0.8

1

0 0.4 0.8 1.2 1 .6C onc entra tio n (m g/L)

Ab

so

rba

nce

Standard Solutions Dissolve 1.000 g of zinc granules in a minimum volume of 1:1 HCl. Dilute to 1 L with 1% v/v HCl to give a 1000 ppm Zn solution.

Ab sorption Se ns itiv ity vs. Fue l flow Rate

0

0.1

0.2

0.3

0.8 1 1.2 1.4 1 .6Fue l Flo w R ate (L/m in)

Ab

so

rba

nce

Interferences

At the 213.9 nm analytical line, non-atomic absorption can cause interference. The use of a background correction method is advised. There are no significant chemical interferences to note.

Ab sorption Se ns itiv ity vs. Lam p C urrent

0

0.1

0.2

0.3

0.4

0 5 10 1 5 20La m p C u rrent (m A)

Ab

so

rba

nce

114


40

Zr

Zirconium



Parameter

Flame: nitrous oxide/acetylene Flame Stoichiometry: reducing HCL Current: 10 mA PMT Voltage: 450 V Slit Width: 0.2 nm Characteristic conc.: 11 mg/L Working Range: 10-2000 mg/L


0

0.2

0.4

0.6

0.8

1


Abso

rban

ce

Standard Solutions Dissolve 1.000 g of zirconium metal in 10 mL of HF and 10 mL of H2O in a Teflon vessel. Slowly add 20 mL of HNO3 dropwise. Allow the reaction to diminish between additions. Dilute to 1 L to give a 1000 ppm Zr solution. Store all solutions in polyethylene bottles.


Interferences Reduce ionization interferences by adding an ionization suppressant (an alkali salt) to all solutions. Oxide formation in the flame is common and results in calibration curve roll-over at higher concentrations. To alleviate this problem and improve calibration curve linearity, add 0.2% Al and 1% HF to all solutions. As well, the interferences can also be reduced by using a more oxidizing flame. However, sensitivity will then be compromised.

115



116



117



118

AI 1200 Cookbook - Book One 7 - Practical Applications FAAS

Chapter 7: Practical Applications - FAAS

• Marine • Water • Biological • Food • Agricultural • Petrochemical • Miscellaneous

119


Application #AA-17

The Determination of Metal Cations in Sea Water by Flame AAS

Classification: Marine Sample matrix: Sea water

Elements of interest:

Ca, K, Li, Mg, Na, Rb, Sr

Introduction This application describes the procedure and methods for determining common metal cations in sea water.

Reagents Needed

Artificial seawater will be made from the following: calcium carbonate, CaCO3; magnesium oxide, MgO; potassium carbonate, K2CO3; sodium chloride, NaCl; (1:1) hydrochloric acid, HCl.

Standard Solutions

Weigh out 0.999 g of CaCO3, 2.974 g of MgO, 1.414 g of K2CO3, and 25.41 g of NaCl. Transfer to a 1 L volumetric flask, and dissolve in a minimum volume of the HCl. Dilute to 1 L with deionized water to give a solution of 10,000 ppm Na, 1250 ppm Mg, 400 ppm Ca, and 400 ppm K.

Sample Preparation

Filter all seawater samples through 0.45 micron Millipore filters. Samples analyzed for Na, Mg, Ca, and K will need to be diluted to be brought into the linear working range. Samples analyzed for Rb, Sr, and Li can be analyzed directly, without dilution.

Guideline Instrument Parameters

Refer to Chapter 6, FAAS Analytical Data Sheets, for guideline parameters for analyzing each element.

120


Application #AA-18

The Determination of Common Metals in Natural Water by Flame AAS

Classification: Water

Sample matrix: Natural water Elements of

interest: Ca, Cu, K, Li, Mg, Mn, Na, Sr, Zn

Introduction This application describes the procedure and methods for determining common metals in natural water samples.

Reagents Needed

2.497 g of dried CaCO3 (calcium carbonate) 1.000 g of copper metal wire 1.907 g of dry potassium chloride (KCl) 5.324 g of lithium carbonate (Li2CO3) 1.000 g Mg metal strip 1.000 g of manganese metal wire 2.542 g of dry NaCl salt (sodium chloride) 2.415 g of Sr(NO3)2 (strontium nitrate) 1.000 g of zinc granules 11.730 g of La2O3 (lanthanum oxide) conc. hydrochloric acid (HCl) and nitric acid (HNO3)

Standard Solutions

All solutions can be used directly as prepared (according to instructions in Chapter 6, FAAS Analytical Data Sheets), expect for Ca and Mg, which require further dilution. To both Ca and Mg solutions, dilute with the 5% w/v La and conc. HCl to give a solution with 0.25% w/v La and 5% v/v HCl.

Sample Preparation

Filter all water samples through 0.45 micron Millipore filters, to remove particulate matter and to avoid blockage of capillary tubing. Prepare a reagent blank for use in the Ca and Mg analyses.


Refer to Chapter 6 for guideline parameters for analyzing each element. For the Ca and Mg analyses, it is required to also measure blank solutions of the reagents used (La and HCl).

121


Application #AA-19

The Determination of Copper and Zinc in Blood Serum and Plasma by Flame AAS

Classification: Biological

Sample matrix: Blood serum, blood plasma Elements of interest: Cu and Zn

Introduction This application describes the procedure and methods for determining copper and zinc in blood serum and plasma samples. Normal levels for Cu and Zn in blood serum are 0.7-1.4 ppm for Cu, and 0.5-1.5 for Zn.

Reagents Needed

1.000 g of copper metal wire 1.000 g of zinc granules 1:1 nitric acid (HNO3), 1:1 hydrochloric acid (HCl) Glycerol

Standard Solutions

Prepare the standards for Cu and Zn according to Chapter 6, FAAS Analytical Data Sheets. The standards are to be prepared in glycerol to create similar viscosity properties in the standards that are present in the samples.

Sample Preparation

Dilute 1 mL of the original blood sample with 5 mL water. Centrifuge the sample for 10 minutes to separate out the protein matter. The supernatant fluid can be used directly for the analysis. Refer to Chapter 6, FAAS Analytical Data Sheets. Guideline

Instrument Parameters

122


Application #AA-20

The Determination of Iron and Nickel in Vegetable Oil by Flame AAS

Classification: Food

Sample matrix: Vegetable oil Elements of interest: Fe and Ni

Introduction

This application describes the procedure and methods for determining iron and nickel in vegetable oil samples. The advantages of this method are that the sample and standard preparations are simple and quick. An alternative method is to char and ash the sample, but it is a tedious and very long process. However, it is a more accurate alternative. Typical levels of Fe and Ni in vegetable oil are 3.8 ppm Fe, and 17 ppm Ni. This method can also be applied to analyze for several other elements, including Cu, Ca, K, Mg, Mn, Na, and Rh.

Reagents Needed

1.000 g of Fe metal strip 1.000 g of Ni metal wire 1:1 nitric acid (HNO3), 1:1 hydrochloric acid (HCl) Glycerol An organic solvent: MIBK, kerosene, acetone, ethanol, or isoamyl acetate/methyl alcohol

Standard Solutions

The preparation of the standards is different from the procedure described in Chapter 6, FAAS Analytical Data Sheets. Sample viscosity is a major factor in these analyses. To compensate for this, the standards should all be made with glycerol to simulate the viscosity properties that are found in the oil samples. 200 ppm Fe standard solution: Dissolve 0.2020 g of dried ferric cyclohexane butyrate/in 5 mL of the organic solvent used to prepare the sample (eg. kerosene) and 2 mL of 2-ethylhexanoic acid). Heat gently to aid dissolution. Make up to 100 mL with the organic solvent. 450 ppm Ni standard solution: Repeat the above procedure, substituting 0.3080 g of dried nickel cyclohexane butyrate for the iron compound. When working solutions are made, be sure to dilute with an appropriate volume of glycerol to simulate the viscosity of the oil sample.

Sample Preparation

Dissolve 10 g of the vegetable oil sample in 100 mL of one of the organic solvents listed above (eg. kerosene).

Refer to Chapter 6, FAAS Analytical Data Sheets. It is required to measure blanks prepared from the organic solvent used (eg. kerosene) and glycerol.


123


Application #AA-16

The Determination of Potassium in Fertilizer by Flame AAS

Classification: Agricultural Sample matrix: Fertilizer

Elements of interest:

K

Introduction This application describes the procedure and methods for determining potassium in fertilizers. Due to the expected high levels of K, the less sensitive analytical line at 404.4 nm will be used. This avoids the need to dilute solutions excessively.

Reagents Needed

Distilled water.

Standard Solutions

Refer to Chapter 6, FAAS Analytical Data Sheets, for information on preparing K standard solutions.

Sample Preparation

Weigh out 2.5 g of the dried fertilizer sample, and transfer it to the 250 mL volumetric flask. Add 200 mL of distilled water to the flask, and bring to a boil. Boil for at least half an hour. Cool the mixture, and then make up to the mark with deionized water. Remove the particulate matter by allowing the mixture to stand overnight or by filtration. Transfer a 25mL aliquot of the liquid to a 100 mL volumetric flask, and make up to the mark. For very concentrated samples, greater dilutions may be necessary.


Refer to Chapter 6, FAAS Analytical Data Sheets.

124


Application #AA-16A

The Determination of various Metals in Plant by Flame AAS

Classification: Agricultural

Sample matrix: Plant Material Elements of

interest: Al, Ca, Fe, K, Mg, Mn, Na, Zn

Introduction

This application describes the procedure and method for determining various metals in plant material digested in nitric and perchloric acids.

Reagents Needed

70% nitric acid (HNO3) and 70% perchloric acid (HClO4) 1.000 g Al metal wire 2.497 g of dried CaCO3 (calcium carbonate) 1.000 g of Fe metal strip 1.907 g of dry potassium chloride (KCl) 1.000 g of magnesium metal strip 1.000 g of manganese metal wire 2.542 g of dry NaCl salt (sodium chloride) 1.000 g of zinc granules

Standard Solutions

All solutions can be used directly as prepared (according to instructions in Chapter 6, FAAS Analytical Data Sheets). Stock standards of each element should be 1000 ppm. Prepare all working standards (from 0.5-40 ppm), in 4% HClO4.

Sample Preparation

Carry out the following acid digestion in a fume hood, and behind a blast shield. Wear appropriate personal protective equipment, including gloves and protective eyewear. Accurately weigh about 0.25 g of the plant tissue in a 19 mm (27 mL) test tube. Put the tube in an aluminum test tube block on a hot plate. Add 1 mL 70% HNO3 and 4mL of HClO4. Heat for 2 hours at 120 °C. Ramp the temperature to 180 °C over 3 hours to drive off the nitric acid. The digestion is complete when white fumes evolve (from the HClO4). It is very dangerous to boil the HClO4 to dryness. Quantitatively transfer the test tube contents to a 25 mL volumetric flask and make up to the mark with water.

Refer to Chapter 6 for guideline parameters for analyzing each element.


125


ELEMENT:Al Ca Fe K

Mg Mn Na Zn

Suggested concentrations of standards (ppm)

Standard #1 Standard #2 Standard #3 5 10 20 10 20 40 2 5 10 5 10 20 5 10 20 1 3 5 1 2 5

0.5 1 2

Calculations

A moisture factor is used in the final calculations to compensate for the mass of water lost from the plant tissue upon heating. The moisture factor is determined by accurately weighing about 2 g of the sample, heating it in an oven at 80 °C for 1 day, and then reweighing the sample.

Moisture factor = (mass after heating)/(mass before heating)

126


Application AA-7

Preparation of Petrochemical Samples for Atomic Absorption Spectrometric Analysis

Introduction: Atomic absorption spectrometry (AAS) has been used in three major areas for the elemental analysis of petroleum products: crude oil, fuels and lubricants, and wear metal analysis [13]. Flame AAS is recommended for the determination of higher concentration metals and graphite furnace techniques for trace metals. Sample Preparation: Three sample preparation methods are currently employed for the determination of metals in petroleum products: 1. Dilution of the petrochemical sample in an organic solvent such as methyl isobutyl ketone (MIBK) or a mixed solvent system such as toluene and glacial acetic acid. 2. Ashing of the sample and dissolution with an appropriate acid. 3. Acid digestion. Dilution Some petrochemical samples can be directly nebulized into a flame or injected into a graphite furnace for analysis. However, when the sample is too volatile, too viscous or the content of the element of interest is too high in the sample, dilution is required. The dissolution of oil/fuel samples in organic solvents is fast and simple. The degree of dilution and choice of solvent will depend on the nature of the sample, the expected level of the element of interest and the background signal appearing during atomization. The volatility of the solvent plays an important role in the selection of drying conditions and the use of an autosampler. Where evaporation is a problem, the sample container should be sealed and the sample dispensed manually and/or a less volatile solvent employed. Mixed solvent and emulsion systems allow the use of inorganic salts for the preparation of standards and calibration curves. Table 7.1 lists a number of organic solvents commonly used in oil/fuel analysis together with their boiling points which can be used as a guide for setting the drying conditions for GFAAS. With regular furnace technique (i.e. not a fast drying furnace technique), most solvents can be dried at a temperature below their boiling point. For a 20 to 40 µl sample, a drying step at temperature setting of 80 to 90% of the boiling point for 30 to 60 seconds has been found to be adequate in most analyses. The dispensing characteristics of the solvent should be studied prior to analysis in order to determine the maximum injection volume. The dispensed volume varies depending on the surface tension of the solvent. Generally injection volumes between 20 and 40 µl are used.

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AI 1200 Cookbook - Book One 7 - Practical Applications FAAS Table 7.1. Solvents commonly used as diluents in AAS

Solvent Boiling point (°C)

Specific gravity

Kerosene 175 - 325 0.78 Tetralin 207 0.97 Shellsol T 186 - 214 0.75 DIBK 166 0.81 Cyclohexanol 161 0.96 3-heptanone 148 0.82 m-xylene 139 0.80 Iso-amylalcohol 132 0.81 MIBK 118 0.79 Toluene 112 0.86 2-methy, propan-1-ol 108 0.78 Isopropanol 83 0.78 Carbon tetrachloride* 77 1.59 Tetrahydrofuran* 65 0.88 Chloroform* 62 1.47

* Not suitable for flame AAS. Ashing of the oil sample The procedure for ashing oil is based on the method developed by Milner, Glass, Kirchner and Yurick [14]. The procedure for the analysis of iron is described as follows. The oil sample (2 g) was treated with concentrated sulphuric acid (2 g) and heated on a hot plate until dry in a vycor crucible. The crucible was transferred to a muffle furnace and ashed at 550°C until all traces of carbon were removed. This was indicated by the absence of a charcoal color and took about 30 minutes. After cooling the sample was treated with concentrated hydrochloric acid (6 ml) and filtered through a Whatman filter paper. The solution was then made up to 25 ml with pure water. This ashing method will give total metal content which represents the sum of particulate and soluble metal. However, volatile metals such as lead and selenium will be lost during the ashing process. Acid digestion Acid digestion of oil samples permits the retention of volatile elements that may be lost during ashing or masked by background interference. During the past years, microwave oven assisted acid digestion has gained increased popularity. This technique makes acid digestion much faster and simpler. References on oil sample digestion with acids can be found in the literature.

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Application Notice #AA-14

The Determination of Silicon by Flame AAS Introduction: Flame AAS with a nitrous oxide/acetylene flame is often a favorable method for the determination of silicon because of its ease of use and high sensitivity. However, the sensitivity and measurement precision vary quite drastically depending on the spectrometer used as well as the flame and chemical conditions. This application notice summarizes the parameters that are critical to the optimization of flame AAS for the analysis of silicon. Important Factors to be Considered: 1. The Flame and its Stoichiometry: The sensitivity for silicon depends significantly upon the stoichiometry of the nitrous oxide/acetylene flame. The best sensitivity is obtained with a fuel-rich flame. For example, using nitrous oxide and acetylene flow rates of 11 and 5.0 L/min. respectively, the absorbance of a 100 ppm silicon solution was about 0.5. However, if the acetylene flow rate was reduced to 4.0 L/min. the absorbance of a 1000 ppm silicon solution might not be detected. The operator must carefully optimize the fuel rates of his or her system to ensure maximum performance. The data supplied by the manufacturer can be used as a reference, however, because many commercially available instruments (other than the AI 1100) rely on a gas flow calibration the values displayed by the instrument may not be the actual flow rates. 2. Bandwidth: The most sensitive line for silicon is at 251.6 nm, however, the silicon spectrum around this line is quite complicated. A narrow bandwidth is essential to resolve this line and thus achieve the best possible sensitivity and most linear calibration curve. It has been proven with experiments that the use of a bandwidth higher than 0.7 nm will significantly decrease the sensitivity and degrade the linearity of the calibration curve. The optimum signal-to-noise ratio and an acceptable linearity can be obtained using a bandwidth of 0.2 nm. Slightly further improvement in linearity is observed with a smaller bandwidth, however, the increased PMT voltage which is required results in poorer signal-to-noise ratios. 3. The Acidity of the Sample: The determination of silicon in a nitrous oxide/acetylene flame is virtually free of interferences, however, the fact that silicon is rapidly precipitated from acid solutions can cause difficulties. It is recommended that any silicon solutions be prepared in basic media. In addition, the burner height and lamp current should also be optimized. It is easiest to use a relatively highly concentrated solution at the beginning of the optimization to ensure a signal can be seen.

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AI 1200 Cookbook - Book One 7 - Practical Applications FAAS Typical Instrument Conditions: Below are some typical conditions for silicon determination with a nitrous oxide/acetylene flame using Aurora Instruments’ AI 1100 Atomic Absorption Spectrometer:

Parameter Suggested Setting Bandwidth 0.2 nm Lamp Current 12 mA Wavelength 251.6 nm PMT voltage 336 V Integration time 5 seconds Acetylene flow rate 5.0 L/min. Nitrous oxide flow rate preset

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AI 1200 Cookbook - Book One References

References 1. Alkemade, C.Th.J. and Herrmann, R., Fundamentals of Analytical Flame

Spectroscopy, Adam Hilger, New York, 1979. 2. L’vov, B.V., Atomic Absorption Spectrochemical Analysis, Adam Hilger, London, 1970. 3. Mavrodineanu, R. and Boiteux, H., Flame Spectroscopy, John Wiley and Sons, New

York, 1965. 4. Price, W.J., Spectrochemical Analysis by Atomic Absorption, Heydon & Son Ltd,

London, 1979. 5. Rains, T.C., Atomic Absorption Spectrometry-Theory, Design and Application, S.J.

Haswell, ed., Elsevier, New York, 1991. 6. Van Loon, J.C., Analytical Atomic Absorption Spectroscopy- Selected Methods,

Academic Press, New York, 1980. 7. Welz, B. and Sperling, M., Atomic Absorption Spectrometry, 3rd. edition, Wiley-VCH,

New York, 1999. 8. Cresser, M.S., J. Anal. At. Spectrom., 8, 270,1993. 9. Holcombe, J.A. and Harnly, J.M., Anal. Chem., 57, 1983, 1985. 10. Williams, R.R., Anal. Chem. 63, 1638, 1991. 11. Smith, S.B. and Hieftje, G.M., Appl. Spectrosc., 37, 419, 1983. 12. Fernandez, F.J., Bohler, W., Beaty, M.M. and Barnett, W.B., Atom. Spectrosc., 2, 73,

1981. 13. Buell, B.E., Appl. At. Spectrosc., 2, 53, 1978. 14. Milner, O.I., Glass, J.R., Kirchner, J.P. and Yurick, A.N., Anal. Chem., 24, 1728, 1952.

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