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ATOMIC ABSORPTION SPECTROPHOTOMETER (AAS)Presentation Author:

G.TEJASRI

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CONTENTS:

History Introduction Principle Instrumentation Interferences and control measures Precautions Data analyzing Applications Limitations High sensitivity techniques Graphite furnace Comparison of AAS with GFAAS and ICP-MS. References

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HISTORY:

The first AAS was presented by Walsh and co-workers in Melbourne in 1954, was a double beam atomic absorption spectrophotometer.

Walsh worked with Perkin-Elmer,

the first AAS instrument developed

by the company was MODEL 303.

Alan Walsh

(1916-1998)

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INTRODUCTION

Atomic Absorption Spectroscopy (AAS) is a quantitative method of analysis that is applicable to many metals and a few non-metals.

Almost every metallic element can be determined quantitatively by using the spectral absorption characteristics of atoms.

It is a very common technique for detecting and measuring concentration of metals in the samples.

It can analyze over 62 elements.

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THE DETECTABLE ELEMENTS ARE THOSE WHICH ARE COLORED IN PINK

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PRINCIPLE:

“When a beam of monochromatic radiation is passed through the atoms of an element, the rate of decrease of intensity of radiation is proportional to the intensity of incident radiation as well as the concentration of the solution.”

This technique basically uses the principle that free atoms (gas) generated in an atomizer can absorb radiation at specific frequency.

The atoms absorb UV or visible light and make transitions to higher electronic levels. AAS quantifies the absorption of ground state atoms in the gaseous state.

The analyte concentration is calculated from the amount of absorption .

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INSTRUMENTATION:

The basic requirements are:

(1) a light source;

(2) a sample cell; and

(3) a means of specific light measurement.

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LIGHT SOURCE:

The two most common line sources used in atomic absorption are :

1. ‘‘Hollow Cathode Lamp(HCL)’’ and

2. ‘‘Electrodeless Discharge Lamp(EDL).’’

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HOLLOW CATHODE LAMP(HCL)

Its an excellent, bright line source for most of the elements determinable by atomic absorption.

Each HCL will have a particular current for optimum performance.

In general, higher currents will produce brighter emission and less baseline noise. As the current continues to increase, lamp life may shorten and spectral line broadening may occur, resulting in a reduction in sensitivity and linear working range.

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HCL–CONSTRUCTION

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CATHODE AND ANODE

The anode and cathode are sealed in a glass cylinder normally filled with either neon or argon at low pressure. At the end of the glass cylinder is a window transparent to the emitted radiation.

The cathode of the lamp frequently is a hollowed-out cylinder of the metal whose spectrum is to be produced. Its constructed from a highly pure metal resulting in a very pure emission spectrum.

The ‘‘multi-element’’ lamp may provide superior performance for a single element or, with some combinations, may be used as a source for all of the elements contained in the cathode alloy.

Anode is made up of tungesten.

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DISADVANTAGES OF HCL:

A finite lifetime – due to depletion of the analyte element from the cathode

Adsorption of fill gas atoms onto the inner surfaces of the lamp – the primary cause for lamp failure

Some cathode materials can slowly evolve hydrogen when heated – a background continuum emission contaminates the purity of the line spectrum of the element, resulting in a reduction of atomic absorption sensitivity and poor calibration linearity.

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ELECTRODELESS DISCHARGE LAMP(EDL)

A small amount of the metal or salt of the element for which the source is to be used is sealed inside a quartz bulb.

This bulb is placed inside a small, self-contained RF generator or ‘‘driver’’. When power is applied to the driver, an RF field is created.

The coupled energy will vaporize and excite the atoms inside the bulb, causing them to emit their characteristic spectrum.

They are typically much more intense and, in some cases, more sensitive than comparable hollow cathode lamps. Hence, better precision and lower detection limits where an analysis is intensity limited.

EDL are available for a wide variety of elements, including Sb, As, Bi, Cd, Cs, Ge, Pb, Hg, P, K, Rb, Se, Te, Th, Sn and Zn.

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CONSTRUCTION OF AN EDL

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PHOTOMETERS

The portion of an atomic absorption spectrometer’s optical system which conveys the light from the source to the monochromator is referred to as the photometer.

Types: single and double beam photometers

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SINGLE BEAM PHOTOMETER:

It is called ‘‘single-beam’’ because all measurements are based on the varying intensity of a single beam of light in a single optical path .

The primary advantage is – it has fewer components and is less complicated than alternative designs.

Hence, its easier to construct and less expensive. With a single light path and a minimum number of optical

components, single-beam systems typically provide very high light throughput.

The primary limitation is – it provides no means to compensate for instrumental variations during an analysis, such as changes in source intensity. The resulting signal variability can limit the performance capabilities of a single-beam system.

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DOUBLE BEAM PHOTOMETERS

It uses additional optics to divide the light from the lamp into a ‘‘sample beam’’ (directed through the sample cell) and a ‘‘reference beam’’ (directed around the sample cell)

The reference beam serves as a monitor of lamp intensity and the response characteristics of common electronic circuitry.

Therefore, the observed absorbance, determined from a ratio of sample beam and reference beam readings, is more free of effects due to drifting lamp intensities and other electronic anomalies which similarly affect both sample and reference beams.

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SINGLE BEAM PHOTOMETER

DOUBLE BEAM PHOTOMETER

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OPTICS AND THE MONOCHROMATOR SYSTEM

An important factor in determining the baseline noise in an atomic absorption instrument is the amount of light energy reaching the photomultiplier (PMT). Lamp intensity is optimized to be as bright as possible while avoiding line broadening problems.

Light from the source must be focused on the sample cell and directed to the monochromator, where the wavelengths of light are dispersed and the analytical line of interest is focused onto the detector.

Light from the source enters the monochromator at the entrance slit and is directed to the grating where dispersion takes place. The diverging wavelengths of light are directed toward the exit slit. By adjusting the angle of the grating, a selected emission line from the source can be allowed to pass through the exit slit and fall onto the detector.

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MONOCHROMATOR

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PRE – MIX BURNER SYSTEM

In ‘‘premix’’ design, sample solution is aspirated through a nebulizer and sprayed as a fine aerosol into the mixing chamber.

Here the sample aerosol is mixed with fuel and oxidant gases and carried to the burner head, where combustion and sample atomization occur.

Fuel gas is introduced into the mixing chamber through the fuel inlet, and oxidant enters through the nebulizer sidearm.

Mixing of the fuel and oxidant in the burner chamber eliminates the need to have combustible fuel/oxidant in the gas lines, a potential safety hazard.

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Only a portion of the sample solution is introduced into the burner chamber by the nebulizer.

The finest droplets of sample aerosol are carried with the combustion gases to the burner head, where atomization takes place.

The excess sample is removed from the premix chamber through a drain which uses a liquid trap to prevent combustion gases from escaping through the drain line.

The inside of the burner chamber is coated with a wettable inert plastic material to provide free drainage of excess sample and prevent burner chamber ‘‘memory.’’

A free draining burner chamber rapidly reaches equilibrium, usually requiring less than two seconds for the absorbance to respond fully to sample changes.

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PRE-MIX BURNER – CONSTRUCTION

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IMPACT DEVICES

Impact devices are used to reduce droplet size further and to cause remaining larger droplets to be deflected from the gas stream and removed from the burner through the drain.

Classified as: impact beads and flow spoilers.

IMPACT BEAD FLOW SPOILER

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IMPACT BEADS

Spherical bead made of glass, silica or ceramic. Normally used to improve nebulization efficiency. Positioned directly in the nebulizer spray as it exits the

nebulizer.

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FLOW SPOILERS

Normally do not improve nebulization efficiency. The primary use is to remove the remaining large

droplets from the sample aerosol. They are used in atomic absorption burner systems

normally are placed between the nebulizer and the burner head.

They typically have three or more large vanes constructed from or coated with a corrosion resistant material.

Smaller droplets are transported through the open areas between the vanes while larger droplets contact the vanes and are removed from the aerosol.

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NEBULIZERS

Classified as :

HIGH SENSITIVITY NEBULIZER STEEL NEBULIZER

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BURNER HEADS AND MOUNTING SYSTEMS

Burner heads typically are constructed of stainless steel or titanium. All-titanium heads are preferred as they provide extreme resistance to heat and corrosion

A ten-centimeter single-slot burner head is recommended for air-acetylene flames.

A special five-centimeter burner head with a narrower slot is required when a nitrous oxide-acetylene flame is to be used.

A ‘‘quick change’’ mount may reduce or eliminate entirely the need for realignment of the atomizer when it is replaced in the sample compartment.

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FLAME SELECTION

Legend:HVG – Hydride GenerationMVU – Mercury Vapor

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STEPS INVOLVED IN FLAME ATOMIZATION

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DETECTOR:

The intensity of the light is fairly low, so a photomultiplier tube (PMT) is used to boost the signal intensity

A detector (a special type of transducer) is used to generate voltage from the impingement of electrons generated by the photomultiplier tube

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PHOTO MULTIPLIER TUBE

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ELECTRONICS

Precision in Atomic Absorption Measurements:

Precision will improve with the period of time over which each sample is read. Where analyte concentrations are not approaching detection limits, integration times of 1 to 3 sec will usually provide acceptable precision. When approaching instrument detection limits where repeatability is poor, precision can be improved by using even longer integration times, up to 10 sec.

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CALIBRATION OF THE SPECTROMETER:

In the linear region, data on as little as one standard and a blank may be sufficient for defining the relationship between concentration and absorbance. However, additional standards are usually used to verify calibration accuracy.

Where the relationship becomes nonlinear, more standards are required for which the accuracy of a calibration depends on the number of standards and the equations used for calibration.

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DATA ANALYZING

By comparing the light intensity that has passed through the sample with that of the same light after it has passed through a blank, the absorbance is measured.

The absorbance of different standard solutions of a compound of the element are also measured and a calibration curve is constructed.

Absorbance is plotted against concentration. We then use the calibration curve to determine the unknown concentration

Concentration (ppm)

Absorbance

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INTERFERENCES & CONTROL MEASURES

NON SPECTRAL

Matrix

Method of Stand

ard Additi

ons

Chemical

add an excess of anotherelement or compound

whichwill form a thermally stable

compound with the interferent

using a hotter flame.

Ionization

adding an

excess of an

element

whichis

very easil

y ioniz

ed

SPECTRAL

Background Absorption

Continuum Source

Background Correction

Zeeman Background Correction

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PRECAUTIONS

Exhaust System: AAS flames produce large amounts of heat & the resultant fumes & vapours may be toxic.

Gas Cylinders: should be located outside of the laboratory in a cool well-ventilated area.

Flammable Solvents: The combination of flame & solvent is a hazardous situation. Always use a solvent with the highest flashpoint consistent with the analysis being conducted. Use covered containers & the smallest practical volume.

Burners: Keep burners clear & do not allow them to block. UV Radiation: Hazardous UV radiation is emitted by

flames, hollow cathode lamps, analytical furnaces. Never look directly at any of these. Operate the AAS with the door or flame shield closed and wear appropriate safety glasses.

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APPLICATIONS:

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LIMITATIONS OF FLAME AAS

Determinations of analyte concentrations in the mg/L concentration region are routine for most elements.

The absorbance depends on the number of atoms in the optical path of the spectrometer at a given instant.

The nebulization process draws sample into the burner chamber at approximately 3-8 ml/min limiting the sample introduction rate, and, therefore, the amount of sample available for transport to the flame.

Only a small fraction of the sample nebulized ever reaches the flame, with the remainder being directed to the drain.

Sample which is introduced into the flame is resident in the light path for only a fleeting moment as it is propelled upwards through the flame.

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HIGH SENSITIVITY TECHNIQUES:

All these limitations in the flame AAS lead to the invent of high sensitivity techniques.

1. The cold vapor mercury technique

2. Hydride generation technique

3. Graphite furnace atomic absorption

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THE COLD VAPOR MERCURY TECHNIQUE

Hg is chemically reduced to the free atomic state by reacting the sample with a strong reducing agent like SnCl2 or NaBH4 in a closed reaction system.

The volatile free Hg is then driven from the reaction flask by bubbling air or Ar through the solution.

Hg atoms are carried in the gas stream through tubing connected to an absorption cell, which is placed in the light path of the AAS.

Sometimes the cell is heated slightly to avoid water condensation but otherwise the cell is completely unheated.

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ADVANTAGES: LIMITATIONS:

Improved sensitivity is achieved through a 100% sampling efficiency (which can be further increased by using very large sample volumes)

All of the mercury in the sample solution placed in the reaction flask is chemically atomized and transported to the sample cell for measurement.

The detection limit for Hg approximately 0.02 mg/L

Its limited to Hg, since no other element offers the possibility of chemical reduction to a volatile free atomic state at room temperature.

The theoretical limit to this technique would be that imposed by background or contamination levels of mercury in the reagents or system hardware.

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HYDRIDE GENERATION TECHNIQUE:

Samples are reacted in an external system with a reducing agent, usually NaBH4.

Gaseous reaction products(volatile hydrides) are then carried to a sampling cell in the light path of the AA spectrometer.

To dissociate the hydride gas into free atoms, the sample cell must be heated.

The cell is either heated by an air-acetylene flame or by electricity

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ADVANTAGES LIMITATIONS:

For As, Bi ,Ge ,Pb ,Sb ,Se ,Sn ,Te the detection limits well below the mg/L range are achievable.

The extremely low detection limits result from a much higher sampling efficiency.

Separation of the analyte element from the matrix by hydride generation is used to eliminate matrix-related interferences.

Its limited to As, Bi ,Ge, Pb, Sb, Se, Sn, Te elements only.

Results depend on the valence state of the analyte, reaction time, gas pressures, acid concentration, and cell temperature.

The formation of the analyte hydrides is suppressed by a number of common matrix components, leaving the technique subject to chemical interference.

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GRAPHITE FURNACE ATOMIC ABSORPTION:

In this technique, a tube of graphite is located in the sample compartment of the AAS, with the light path passing through it.

A small volume of sample solution is quantitatively placed into the tube, normally through a sample injection hole located in the center of the tube wall.

The tube is heated through a programmed temperature sequence until finally the analyte present in the sample is dissociated into atoms and atomic absorption occurs.

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ADVANTAGES

Detection limits fall in the ng/L range for most elements. The sample is atomized in a very short period of time,

concentrating the available atoms in the heated cell and resulting in the observed increased sensitivity.

This uses only micro litre sample volumes, which is compensated by long atom residence time in the light path.

Much more automated than the other techniques. Wide applicability. It can determine most elements measurable by AA in a

wide variety of matrices.

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GRAPHITE FURNACE TECHNIQUE

It’s a non flame technique Main components:

1. The atomizer

2. The power supply and

3. The programmer The atomizer is located in the sampling compartment of

the atomic absorption spectrometer, where sample atomization and light absorption occur. The power supply controls power and gas flows to the atomizer under the direction of the programmer, which is usually built into the power supply or spectrometer

Graphite tube

• Electrical contacts

Enclosed water cooled housing

• Inert purge gas controls

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LONGITUDINALLY-HEATED GRAPHITE FURNACE ATOMIZER

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A GRAPHITE TUBE FOR A TRANSVERSELY-HEATED FURNACE.

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The power supply and programmer perform the following functions:

1. Electrical power control

2. Temperature program control

3. Gas flow control

4. Spectrometer function control

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STEPS MAKE UP THE TYPICAL GRAPHITE FURNACE PROGRAM

1) Drying

2) Pyrolysis

3) Cool down (optional)

4) Atomization

5) Clean out

6) Cool down

Legend:

1) Temperature: final temperature during step

2) Ramp time: time for temperature increase

3) Hold time: time for maintaining final temperature

4) Internal gas: gas type and flow rate

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COMPARISION WITH OTHER TECHNIQUES:

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Parameters AAS GFAAS ICP – MS

Temperature 2300°C – 2700°C 3000°C 6000°C

Radiation used UV ,VISIBLE UV, VISIBLE ----

Detection limit Ppm Ppb Ppt

Elements applicable to

68+ 50+ 82

Sample throughput 10-15 sec per element 3-4 min per element All elements <1 minute

Sample volume required

Large Very small Very small to medium

Isotopic analysis No No Yes

typical consumableitems and utilities required

acetylene/nitrous oxide gases(compressed air source)hollow cathode lampsreagents and standardspower

argon gashollow cathode lampsgraphite tubes and conesreagents and standardspowercooling water

argon gasquartz torchessampling and skimmer conesreagents and standardspump tubingpowercooling water

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REFERNCES

Shriver and Atkins’ Inorganic Chemistry, Fifth Edition Concepts, Instrumentation and Techniques in Atomic Absorption

Spectrophotometry by Richard D. Beaty and Jack D. Kerber An elementary overview of elemental analysis by thermo elemental

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Queries

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