Unit 11 Applications of AAS and AES

8/9/2019 Unit 11 Applications of AAS and AES

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Applications of AAS

and AESUNIT 11 APPLICATIONS OF AAS AND AES

Structure

11.1 IntroductionObjectives

11.2 Salient features of AAS and AESSalient features of AAS

Salient features of AESComparison between AAS and AES

11.3 Sample Preparation11.4 Applications of AAS

Biological Samples

Environmental Samples

Industrial Samples

11.5 Applications of AESBiological Samples

Geological SamplesEnvironmental Samples

Industrial Samples

11.6 Summary

11.7 Terminal Questions

11.8

Answers

11.1

INTRODUCTION

In the preceding Units 9 and 10, you have studied about basic principles and

instrumental aspects of atomic absorption spectrophotometry (AAS) and atomicemission spectrometry (AES) respectively. You would recall that atomic absorptionspectrophotometry concerns the absorption of radiation by the atomised analyte

element in the ground state; the atomisation being achieved by the thermal energy ofthe flame or electrothermally in an electrical furnace. The wavelength(s) of the

radiation absorbed and the extent of the absorption form the basis of the qualitative

and quantitative determinations respectively. On the other hand atomic emissionspectrometry concerns the emission of radiation by the suitably excited atomic

vapours of the analyte; the atomisation as well as the excitation being achieved by anyof the numerous available energy sources such as flame arcs, sparks, or plasmas. Here,

the emitted radiation and its intensity form the basis for the qualitative and quantitativeapplications of the technique. You have also learnt about flame emission

spectrophotometry (FES), another atomic emission technique, commonly called as

flame photometry in Unit 7. You would recall that it is a simple, rapid and inexpensivemethod for routine analysis of alkali and alkaline earth metals like, sodium, potassium,lithium, calcium and barium in environmental, clinical and biological samples

especially in biological fluids and tissues.

In this unit, we take up some of the important applications of atomic absorption

spectrophotometry and atomic emission spectrometry. We will begin the unit with

recalling the salient features of the two techniques and then take up the applications ofAAS and AES. In the next block you would learn about some miscellaneous

spectroscopic methods.

Objectives

After studying this unit, you will be able to:

• outline the salient features of atomic absorption spectrophotometry and atomic

emission spectrometry,


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Atomic SpectroscopicMethods-II • compare and contrast atomic absorption spectrophotometry and atomic emission

spectrometry,

• enlist different areas of applicability of atomic absorption spectrophotometry

and atomic emission spectrometry,

• discuss the merits and limitations of atomic absorption spectrophotometry and

atomic emission spectrometry,

•

describe some representative applications of atomic absorption

spectrophotometry and atomic emission spectrometry,

• rationalise the complementary nature of atomic absorption spectrophotometry

and atomic emission spectrometry.

11.2

SALIENT FEATURES OF AAS and AES

You know that in atomic spectroscopy, the element present in a sample is converted to

gaseous atoms or elementary ions in a process called atomisation which may be

brought about by any of the available methods. The absorption of the radiation by thevapourised atoms in the ground state, or emission or fluorescence emission of suitably

excited state forms the basis of different types of atomic spectroscopies. Collectively,

the atomic spectroscopic methods can be used for the qualitative and quantitative

determination of about 70 elements in a wide variety of samples of clinical, biological,and environmental origin. You have learnt in details about AAS and AES in Units 7,9

and 10. Let us recollect the salient features of the AAS and AES methods before

taking up their applications.

11.2.1

Salient Features of AAS

• AAS concerns the absorption of characteristic analyte radiation by the atomisedanalyte element in the ground state. The wavelength(s) of the radiation absorbed

and the extent of the absorption form the basis of the qualitative and quantitativedeterminations respectively.

• In flame atomic absorption spectrophotometry, either an air-acetylene or a

nitrous oxide-acetylene flame is used to evaporate the solvent and dissociate the

sample into its component atoms.

• It not an absolute method of analysis; the routine quantitative determinations

using AAS are based on calibration method. In addition, the internal standard

method and standard addition methods are also employed.

•

Compounds of the alkali metals, some of the heavy metals such as lead or

cadmium and transition metals like manganese or nickel are all atomised withgood efficiency by flame However, a number of refractory elements like V, Zr,Mo and B do not perform well with a flame source.

•

Graphite furnace atomic absorption spectrophotometry (GFAAS) in which theatomisation is achieved electrothermally, is a much more sensitive method as

compared to flame AAS. The higher atom density and longer residence time in

the graphite tube improve furnace AAS detection limits by a factor of up to1000 compared to flame AAS. The detection limits may extend to the sub-ppb

range.

• GFAAS requires a very small sample size and does not require any sample

preparation; even solid samples can be analysed without dissolution.

• The background absorption effects in GFAAS are managed by diluting the

sample or selecting another resonance wavelength line. In matrix modifier

method a reagent is added to the sample that may modify the matrix behaviourand thereby tackle the problem of background.


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Applications of AAS

and AES• Accuracy in AAS method is generally of the order of 0.5 – 5%; the precision

being 0.3 – 1% at absorbance larger than 0.1 or 0.2 for flame atomisation and

1 – 5% with electrothermal atomisation.

• It is a robust technique that employs easy to use equipment and can achieve

good detection limits. As the turnaround time is small the cost of analysis per

sample is not much. However, lack of automation, and usage of flammable

gases are not desirable.

11.2.2

Salient Features of AES

• In atomic emission spectrometry (AES) a reproducible and representative

amount of the sample is introduced into an atomization-excitation source

wherein it is converted into atomic vapours of the analyte in excited state. Theradiation emitted from these is characteristic of the constituents of the sample.

• The AES is a versatile method due to the availability of a wide range of

atomization-excitation sources. Currently, plasma is the most used source. It is

high energy source which is an electrically neutral conducting gaseous mixturehaving a significant concentration of cations and electrons.

• The plasma can be sustained by supplying energy from a suitable externalsource. Depending on the power sources employed, there are three different

types of plasmas. These are, the inductively coupled plasma (ICP), the directcurrent plasma (DCP) and the microwave induced plasma (MIP). These plasmasuse radiofrequency, direct current and microwave radiation respectively as the

power sources.

• As the energy of the plasma source is quite high, it ensures the excitation of the

atoms of all the elements present in the analyte which emit EM radiationcharacteristic of different elements. Thus, it is a multielement technique.

• Argon gas is commonly employed as plasma gas due to its inertness, simple

emission spectrum, moderately low thermal conductivity, and good naturalabundance.

•

Two types of spectrometers are used for ICP-AES. These are sequential

spectrometers and simultaneous spectrometers depending on whether theemitted radiation is measured sequentially or simultaneously.

• In ICP-AES the spectral interference due to the line-rich spectra of the hot

plasma source can be minimised by using high resolution spectrometers or using

an alternative analyte line. The background effects require the use of offlinebackground correction techniques, or by moving to an unaffected analyte line.

The matrix effects are generally handled by using internal standard method.

• The ICP spectrometers are, however, relatively expensive to purchase and

difficult to operate as the user requires extensive training for the maintenance ofthe instrument.

11.2.3

Comparison between AAS and AES

As has already been emphasised, AAS and AES have become the mainstay of theanalytical techniques for major, minor and trace element analysis in geological,

biological, environmental and industrial samples. Both the techniques can be used for

the determination of more than sixty elements, many of which can be determined at1 ppm level. As regards their applicability, these two techniques are complementary to

each other though several points are common amongst them.

It must be kept in mind that only metals and metalloids can be determined by usualflame methods like FAAS. This is because resonance lines for nonmetals fall in


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Atomic SpectroscopicMethods-II vacuum UV region though some indirect methods have been developed for the same.

For example, chloride can be determined by precipitation with Ag+ and then either theexcess of Ag

+ or the one which has already reacted is measured. Similarly phosphorus

(525.9 nm) and sulfur (383.7 nm) species exhibit sharp molecular band emission in the

Ar-H2 flame. Generally, AAS is considered as more sensitive technique at wavelengths

< 300 nm, whereas in visible region, AES is more advantageous. Some elementsexhibit maximum sensitivity using molecular band emissions.

As the source of radiation in AAS is a hollow cathode lamp which emits thecharacteristic radiation of a given element, it is a unielemental technique. It is not

convenient to measure more than one element at a time by AAS as it is difficult to

incorporate more than a single source into the system. Each hollow cathode lampemits efficiently the spectrum of only one, two, or three elements at a time, measuringadditional elements requires substituting a new hollow cathode lamp. Though some

advances have been made in continuum source atomic absorption spectrophotometry

yet these arrangements are somewhat limited as sources extending into the ultravioletregion of the spectrum are not widely available.

The basic principle of graphite furnace atomic absorption spectrophotometry(GFAAS) is essentially the same as flame atomic absorption spectrophotometry, the

only difference being that the atomisation is achieved in a small, electrically heated

graphite tube, or cuvette, which is heated to a temperature up to 3000°C to generatethe cloud of atoms. The higher atom density and longer residence time in theelectrothermal tube improve the detection limits by a factor of up to three orders of

magnitude as compared to flame AAS and we can go down to the sub-ppb range.However, the use of graphite cuvettes, do not sort out the issue of determiningrefractory elements.

It is essential that the AAS instrument should always be calibrated by preparing atleast four standard solutions over the concentration range of interest and measuring the

absorbance under the same experimental conditions. The correction, if necessary,should be applied to the calibration plot. Sometimes, the method of standard addition

is used to compensate for chemical and other interferences.

In contrast to atomic absorption spectrophotometry, atomic emission spectrometry isinherently a multielement method. Especially the high temperature of plasma ensureseffective atomisation and lead to intense atomic emission. The emission occurs from

all elements at the same time and is isotropic. The simultaneous multielementdeterminations can be made simply by using a multichannel detection system.Multichannel devices using two dimensional spectral dispersion along with two

dimensional arrays of detector elements offer extremely good sensitivity and low

noise.

More so at the operating high temperatures of ICP torch, even the most refractoryelements are atomised with high efficiency. As a result, detection limits for these

refractory elements can be of the orders of magnitude lower with ICP than with FAAStechniques. These may be at the 1-10 parts per billion level. We can safely generalise

the order of detection limits of different techniques as GFAAS (sub-ppb) > ICP-AES(1-10 ppb) > FAAS (sub-ppm).

Further, the dynamic range of the various techniques is also important, as it directly

affects the amount of dilution required in preparing solutions for analysis. If the

dynamic linear range is quite wide, we may use fewer standards. The dynamic rangesof FAAS and GFAAS are of the order of only 10

2-10

3 only whereas the same for ICP-

AES the dynamic range spreads upto 106. This makes it a suitable technique that is

capable of measuring from trace to percent levels. A comparative account of thecharacteristics of AAS and ICP-AES are briefly summarised in Table 11.1.


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Applications of AAS

and AESTable 11.1: A comparative account of the characteristics of AAS and ICP-AES

Atomic Absorption Spectrophotometry ICP-Atomic Emission Spectrometry

Primarily a single element technique; thoughsome instruments with multielement sourcesare available.

Principally a rapid and multi-elementtechnique.

The flame constituents contribute to thespectral, background and chemical

interferences.

Plasma is an optically thin emission sourceand is relatively free from chemical

interferences.

The dynamic range is spread over three

orders of magnitude for FAAS and twoorders for GFAAS.

The dynamic range is large and extends over

a range of 4 to 6 orders of magnitude. It issuitable for analytes from parts per billion to

99.9 per cent.

For AAS the detection limits are in the rangeof ppm whereas these may go down to sub-

ppb level for GFAAS.

Detection limits are generally very low: 1 to

100 ng/g or µg/L (parts per billion).

Flame AAS is easy to set up and to use, and

requires minimal operator skills, the GFAASon the other hand is considerably more

difficult to operate.

It falls between these two AAS techniques;

however, it is a bit easier to master thanGFAAS.

FAS procedure cannot be automated

whereas it is possible to automate GFAAS.

ICP-AES measurements can be automated.

The accuracy is not very promising. Good accuracy and precision (relativestandard deviation about 1 per cent).

The AAS determinations using flame arerapid and precise and are applicable to about67 elements.

It can be used for the determination of mostelements except Ar. In practice,approximately 70 elements can be

determined.

Not suitable for the elements like, B, C, Ce,

La, Nb, Pr, S, P, Ti, Ta, V and Zr.

The elements that are difficult to be

determined by AAS, can be measured by

AES.

11.3

SAMPLE PREPARATION

All samples for determination by AAS or AES must be in solution form except forspark source AES where solids especially metals and alloys with smooth surface canbe analysed directly. The detailed procedures for sample preparations have been

discussed in Section 9.7 and subsection 10.4.2 respectively for AAS and AES. You

would recall that in principle, the sample in solid, liquid or in the gas phase can beanalysed by flame AAS but in practice the sample is taken in the solution form. The

solution of the solids is generally prepared by wet dissolution method using a suitableacid. The presence of organic solvents of low molar mass e.g. alcohols, ethers, ketones

and esters are found to enhance absorption peaks and hence increase sensitivity. A

microwave digestion system (MDS) offers more rapid and efficient decomposition ofcomplex matrices of geological and biological samples. It greatly reduces the operator

time to prepare samples for analysis. More so, it can be easily automated also.

On the other hand in ICP-AES, the solution preparation depends on the nature of the

sample and the concentration of elements to be determined. The solution for ICPanalysis can be prepared either by wet acid method or by direct attack method and

suitable precautions are taken as per the requirements of the plasma source.


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Atomic SpectroscopicMethods-II It must be noted that the possible contamination during dissolution and at the

workplace is the most important source of error in the analysis of trace elements andmust be avoided. Contamination may come from the air, from the skin of the subject/

sample collector, additives and reagents used in the analysis, as well as parts of

instrumentation including glass or plastic wares. Biological materials of human and

plant origin must be handled with extreme care because of sample inhomogeneityespecially for trace element analysis. Body fluids such as blood, viscera, urine, etc.additionally need stabilization and homogenization so as to avoid occurrence of any

changes in their composition, prior to actual analysis. It is also advisable to keep thetotal number of transfers to a minimum, and to use accessories made of non-wettable

and inert materials.

SAQ 1

What precautions should be observed while preparing samples for AAS and AES?

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11.4

APPLICATIONS OF AAS

Atomic absorption spectrophotometry is now a routinely and widely employedtechnique for trace and ultratrace analysis of complex matrices of geological,biological, environmental or industrial origin. The atomic absorption methods using

flame are rapid and precise and are applicable to about 67 elements. Electrothermalmethods of analysis on the other hand are slower and less precise; however, these are

more sensitive and need much smaller samples. Let us take up some applications of

AAS in different areas. We begin with biological samples.

11.4.1 Biological Samples

A wide range of the samples of biological origin are subjected to analytical procedures

for the determination of the elements present in them. These may include plant leaves,fruits, vegetables, blood, urine, muscle tissue, hair, etc. The major difficulty in the

analysis of these materials is their complex nature. More so, these samples cannot beanalysed directly but require dry ashing followed by wet digestion with oxidising

acids such as HNO3 and HClO4. In case of blood analysis, plasma or serum isgenerally preferred because of the presence of significant amounts of clinicallysignificant elements in them.

i) Determination of calcium in serum

A typical AAS determination of calcium in serum is carried out by calibration method

wherein, a calibration plot is obtained by measuring the absorption of characteristic

radiation (422.67 nm) for a series of standard solutions of calcium in a similar matrix.An air-acetylene flame is used with a premix burner. The normal calcium content of

serum is generally about 100 ppm and it is determined by diluting the sample 1:20with 1% SrCl2 solution. Thus, a typical sample would contain about 5 ppm of Ca.

Therefore, an equivalent amount of sodium and potassium are added to the standardsolutions. The plot is then used to determine the concentration of the given sample.

It needs to be mentioned that the effects of instrumental parameters and of phosphateand aluminium ion on calcium absorption are to be suitably accounted for an effectivedetermination. Instrumental parameters such as burner height and fuel air ratio may be

studied to optimise the experimental conditions of flame height and fuel gas pressure.

Serum is the supernatantliquid of the clotted blood

and is separated bycentrifugation after

addition of anticoagulant

such as heparin.


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Applications of AAS

and AESSimilarly the effect of organic solvent such as ethanol may also be studied. The effectsof interferants is borne by using 5 ppm each of phosphate, sodium and aluminiumsolutions

In an alternative determination, using the method of standard addition a series of

calcium standards of 0, 2.5, 5, 7.5, 10 and 15 ppm are prepared from the 50 ppm stocksolution and SrCl2 is added to standards and the unknown to give a final concentrationof 1%. The standard addition absorbance calibration plot is then prepared and used for

the determination of the concentration of analyte sample.

ii)

Determination of cadmium

Cadmium is one of the most important toxic elements from the environmental point ofview. It occurs in nature mainly due to volcanic activity. It is used in plating of metals,

as stabiliser in polyvinyl chloride, pigments, Ni-Cd batteries and alloying. It is the

prime cause of ‘itai-itai’ disease first observed in Japan. Cadmium along with lead hasbeen the most studied element with regard to human toxicology as it has no role inhuman or plant nutrition. It is highly toxic even in trace amounts to the human body.

Total intake of cadmium in Germany, USA and most European countries is in the

range, 10-30 µg/day whereas in contaminated areas of Japan, its intake is as high as400 µg/day. It is most likely to be ingested by tobacco smoking especially cigarettes.

Absorption of cadmium is higher in females than in males though its transport in the

intestinal tract is influenced by the presence of various food components such asproteins and amino acids.

Cadmium in blood may be used as a biological monitoring measure for recentoccupational/environmental exposure. In addition, cadmium in urine may also be used

as a measure of biological monitoring for body burden where it reflects the totalaccumulation of cadmium in the body. Typically it occurs at ≤ 1µg/L in the blood ofhealthy and nonexposed nonsmokers in various countries. Considering the

requirements of detection limit and contamination free sample handling, graphite

furnace atomic absorption spectrophotometry is the method of choice where thedetection limit is 0.04 µg/L.

A typical determination of cadmium in blood involves de-proteination with nitric acidfollowed by direct determination by GFAAS using source with 228.8 nm output. The

blood sample is collected in plastic collection tubes using vinyl gloves free of talc and

is stored at a temperature of – 20oC to 4

oC. All the laboratory ware is to be soaked in

diluted nitric acid and biodistilled quality water is used for dilution work. The

determination is preceded by the obtaining calibration curves using matrix adaptedcalibration solutions. In simple words it means that the calibration solution contains all

the known components of the analyte sample. A multiple standard calibration ispreferred. Similarly, Cd could also be determined in urine, hair and other body tissues.

iii)

Determination of lead

As you know, lead is another highly toxic element which is an environmental

contaminant. It enters into biological systems like plants and animals and reaches

blood, urine, teeth, bones, hair, plant and animal tissues, etc. These materials need to

be analytically assessed for the amount of lead so that its damage potential can beascertained. From the viewpoint of occupational and environmental toxicology the

determination of lead in blood is the most important since the concentration of lead inwhole blood is considered to be the best indicator of current lead exposure in humans.

It enters into human blood because of inhalation of polluted air, food and water though

these are less relevant for assessing health hazards for humans than the amount of lead

actually absorbed.

In a typical lead determination, after adding heparin, a natural anticoagulant, the blood

is treated with trichloroacetic acid to precipitate proteins. These are then separated by

Quality control is usually

carried out by using

certified reference

materials from NIST

(USA) and NIES (Japan).


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Atomic SpectroscopicMethods-II centrifugation. In order to avoid interferences, lead is extracted in an organic solvent

methyl isobutylketone (MIBK) after adding ammonium pyrrolidinedithiocarbamate(APDC) at pH 3. The lead is extracted as Pb (APCO)2. The organic phase is then

aspirated into air-acetylene flame for the determination of lead. The detection limit of

lead in blood is 0.1 µg/mL. Most values of lead in blood are in the range 0.3– 0.4

µg/mL with 0.6 µg/mL as the upper limit. It is essential that internal and externalquality control should be used for the determination of lead in blood.

iv) Zinc in plant leaves

Zinc is an essential nutrient in plants and remains distributed in all parts of the plant.About 1g dried plant leaves are grounded with pestle and mortar and dry ashed insilica crucible at 500ºC. The ash is then dissolved in acid and final solution is prepared

to 0.1 M HCl. The solution is then directly aspirated into an air-acetylene flame ofAAS. A blank is also prepared in exactly similar manner.

11.4.2 Environmental Samples

Air, soil and water are three components of environment where determination of toxiccontaminant is of extreme importance. Analysis of particulate matter in air from

industrial establishments is the most representative study of environmental samples byatomic spectrometry.

i)

Analysis of airborne particulate matter

In the analysis of airborne particulate matter, the choice of sample collection locationand collection procedure is very important. For example samples may be collected

from surrounding areas of a factory emitting harmful gases affecting workers healthadversely or a residential colony located near industrial establishment where toxic

pollutants may travel and thus affect general public. A measured volume of air is

collected on a cellulose acetate membrane filter using air sampler. Weighed amount ofparticulate matter is scratched out of the filter paper or the filter paper itself may be

dry ashed in a low temperature furnace so as to avoid loss of volatile elements. Theparticulate matter or ash is then dissolved using acid digestion method and heating on

hot plate. The final solution is prepared in dilute HCl and making up final volume to afixed volume. Appropriate hollow cathode lamps are selected depending on the

elements to be determined and respective standard solutions are prepared. Calibrationplots should be drawn for each element to be determined and the test solution isaspirated. Thus concentration of desired elements in air dust particulate matter may bedetermined. Results are usually reported in terms of µg/m3 of air.

ii) Mercury in air/water

Metallic mercury is important as it forms amalgam with other metals. Its alloy withsilver was earlier used by dentists for dental filling though it is no longer so because of

toxicological effects known since many years. Mercury in air is collected by bubbling

air through an acidic KMnO4 solution where volatile elemental mercury is trapped byoxidising it to Hg

2+. The excess permanganate is reduced with hydroxylamine, and the

collected mercury (or mercury in a water sample) is then reduced to the element by

SnCl2 according to following equations.5Hg

0 + 2MnO

– 4 + 16H

+ → 5Hg

2+ + 2Mn

2+ + 8H2O

6MnO–4 + 5NH2OH + 13H+ → 6Mn

2+ + 5NO–3 + 14H2O

Hg2+

+ SnCl2 + 2Cl–

→ Hg0 + SnCl4

As elemental mercury has appreciable vapour pressure at room temperature, and bybubbling argon through the solution, mercury vapour is swept into a quartz ended cell

where its atomic absorption is measured at 253.6 nm using mercury line. A calibrationcurve should be prepared before the sample is run. At least two blanks should also be


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Applications of AAS

and AESprepared in the same manner, omitting the addition of mercury. The measuredabsorbance is corrected for the blanks and the amount of mercury is determined fromthe calibration curve.

Similarly, the water sample from tap, river, or other sources can be analysed. Tap

water should contain around 1 ppb or less mercury. In such determinations the watersamples and the standards should be run in a similar manner. As in the case of airsamples, the correction should be made for the reagent blank as its magnitude will

generally govern the detection limit of the procedure. Extreme care must be taken tominimise reagent and glassware contamination. From the calibration graph the

quantity and concentration of mercury in the sample is determined.

iii) Trace element contamination in soil

In order to determine toxic contaminants in soil samples, the representative samples

are collected from surface of the soil, and also at some depth. These are then passed

through a sieve to make it uniform sample and is stored in separate containers to avoid

cross contamination. In order to determine all the contaminants, the sample is preparedby treating a weighed amount of soil with 1:1 nitric acid and making appropriate

solution. Metal contents such as Ni, Cu, Zn, Cd, Pb, etc. are then determined by using

appropriate hollow cathode lamp and air-acetylene flame. It is essential that thestandard solutions for each element are prepared in appropriate concentration range

and their respective calibration plots are obtained.

11.4.3

Industrial Samples

Quality control of finished products of steel industry and other products such as alloysrequires accurate analysis. For such an analysis an alloy or steel is be dissolved in acid

(HCl, HNO3, HClO4 or a mixture of these) and a solution is prepared for analysis byAAS. Care must be taken to eliminate excess of acid. A typical example is described

in following lines.

i) Determination of molybdenum in steel

Weighed amount of sample is dissolved in aqua regia and finally in dil HCl. Finalvolume is made up to fixed volume in a volumetric flask by adding doubly distilled

water. Molybdenum can be determined in acetylene-air or acetylene-N2O flameselecting a wavelength of 313.26 nm. Let us take up an example.

Example

0.32 g stainless steel sample was dissolved in nitric acid and the resulting solution was

made to 100 cm3 with water. Five standards and the sample solution were aspirated

into flame for the determination of nickel. The following observations were made.

Concentration of Nickel (ppm) Absorbance

2 0.126

4 0.250

6 0.374

8 0.500

10 0.626

Sample 0.226

Calculate the percentage of nickel in steel sample.


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Atomic SpectroscopicMethods-II Solution

Let us prepare the calibration plot by taking the concentration on X-axis andabsorbance on Y-axis, as shown below

According to the calibration plot the sample concentration is found to be 3.612 ppm. It

corresponds to 1.12%.

ii) Tin in canned fruit juice

Availability of fruit juice in tinned cans is becoming increasingly common though it is

also marketed in cartons. As tin is likely to be leached in acidic medium of juice the

contents of the can get contaminated. Hence its determination is quite important toascertain the contamination, if any. Sn can be determined successfully by graphitefurnace atomic absorption spectrophotometry (GFAAS). In a typical determination,

the juice solution is prepared in dil HCl and boiled till it becomes clear. In case the

solution is still turbid or colloidal then it is centrifuged and only supernatant is takenfor analysis. As the concentration of Sn in juice is likely to be very low, standards are

prepared in the concentration range 25-100 ppb together with acid as blank. The

measurement is made at a wavelength of 224.6 nm.

SAQ 2

Enlist some important applications of AAS in the area of environmental analysis.

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11.5

APPLICATIONS OF AES

Emission spectroscopy is widely used for qualitative as well as quantitative analysis

because of high sensitivity and the possible simultaneous excitation of many elements,

notably metals and metalloids. AES is especially suited for rapid survey analysis ofthe elemental contents in small samples at level of 10µ g/g or less. It is essential to

construct an analytical curve with known standards. Often the ratio of analyte

emission intensity to the emission intensity of a second element contained in, or addedto the sample is used. This internal standard method improves the precision of

analysis.

ICP-AES is used widely for determining trace metals in environmental samples suchas drinking water, industrial waste water and ground water supplies and so also for


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Applications of AAS

and AESdetermining trace metals in petroleum products, biological materials, foodstuffsgeological samples and industrial quality control. The simultaneous multielementaldeterminations make it possible to form correlations and to reach conclusions that are

impossible with single element determinations. The excellent sensitivity and wide

working range for many elements together with the low level of interferences make

ICP-AES a nearly ideal method. Let us learn about some typical analyticaldeterminations in different areas using AES.

11.5.1 Biological Samples

As you have learnt above, a wide range of the samples of biological origin are

subjected to analytical procedures for the determination of the elements present in

them. Let us take up the determination of sodium in serum as a representative example

of the application of AES in biological samples.

Determination of sodium in serum

Determination of sodium in water or serum is carried out by following the

characteristic emission at 589 nm. A calibration plot is prepared between emissionintensity and concentration of the standard solutions. The concentration of the sample

solution is then determined from the calibration plot. In some of the determinations aknown amount of an internal standard like lithium is added to the standard solutions as

well as the sample solution. The calibration curve is drawn between the emissionintensity ratios of the characteristic emissions of sodium to lithium versus the

concentration of the standard solutions of sodium.

11.5.2 Geological Samples

The analysis of geological samples constitutes a major area of the application of

atomic emission spectrometry. A large proportion of the elements of the periodic table

present in the geological samples can be conveniently determined by ICP-AESspectrometry. These are now routinely being measured well within the limits of the

methods. In past, a number of analytical methods have been described for the

determination of a particular element in a given sample type. These methods could notbe used in a generalised way for samples with analytically different matrices. It

became difficult to determine the other elements present in the matrix. More so, the

elements that are readily detected in mineralised rock samples may not be detectablein non-mineralised samples such as water. However, when ICP-AES is used for

analysis of normal silicate rocks, the range of elements that can be measured is large.

Only a few of the elements present at concentration above the 10 µg g-1

level are notreadily determined by routine ICP analysis.

You might know that in the context of rock analysis, there are ten elements that are

conventionally quoted as oxide equivalents. These are Si, Al, Fe, Mg, Ca, Na, K, Ti,

Mn, P; these can be determined without difficulty. In addition, many of the traceelements such as Li, Sr, Ba, Sc, Y, La, Zr, V, Nb, Cr, Co, Ni, Cu and Zn, that aredetermined in a routine trace analysis programme can also be conveniently measured

by ICP analysis. The detection limits for some typical elements by ICP are compiled

in Table 11.2.

Further, the trace elements including Mo, Ag, Cd and Hg in mineralised geological

samples can readily be determined when they occur at levels above ‘background’concentration. Lead can also be measured in mineralised samples but not as well at

normal levels in silicates (below 20-40 µg/g). The detection limits for Sn, W, U and

Th are not good, but concentrations above 50 µg/g can be measured. It is also possible

to determine the rare earth elements down to sub µg/g level in a rock sample using aconcentration technique.


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Atomic SpectroscopicMethods-II Table 11.2: Detection limits (µg/ml) of different elements by ICP-AES

Element Detection limit Element Detection limit

Ag 0.004 Mo 0.0001

Al 0.00008 Na 0.00002

As 0.002 Ni 0.0001

Au 0.04 P 0.015

B 0.0001 Pb 0.001

Ba 0.00001 Pd 0.0008

Be 0.000003 Pt 0.08

Ca 0.0000001 Rh 0.003

Cd 0.0002 Sc 0.003

Ce 0.0004 Se 0.03

Co 0.003 Si 0.01

Cr 0.0008 Sn 0.003

Cu 0.0006 Sr 0.00003

Fe 0.00009 Th 0.003

Ga 0.0002 Ti 0.00003

Hf 0.01 Tl 0.2

Hg 0.01 U 0.03

In 0.03 V 0.00006

La 0.001 W 0.0007

Mg 0.000003 Zn 0.00001

Mn 0.00002 Zr 0.00006

11.5.3

Environmental Samples

Environment is an important area wherein the elemental analysis is of significant

importance. Let us take up the analysis of trace elements in airborne particulate matter

by AES as a representative example of this group.

Trace elements in airborne particulate matter

AES has been used extensively for the determination of trace elements in atmospheric

particulates, especially large scale survey studies where simultaneous multielementanalysis is required. Airborne particulate matter is routinely collected by drawing a

measured volume of air through filter material such as fiberglass, asbestos, cellulose

paper, porous plastic, or graphite in the form of discs or electrodes. However, for the

determination of trace elements, the chemical composition of filter is important. Forexample glass filters show high concentrations of Ba, Sr, Rb, Zn, Ni, Fe, Ca, As and

other elements. The composition of filter materials is particularly significant insampling relatively clean atmospheres because of the low particulate levels collectedin reasonable sampling time.

The particulates are collected, dried, and weighed; then spectroscopic buffer is added

along with internal standards. The sample so prepared is then suitably determined. The

detection limits between 0.1 and 5 ng can be obtained for up to 14 elements.


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Applications of AAS

and AES11.5.4 Industrial Samples

The application of atomic emission spectroscopy in analysing the industrial samplescan be illustrated by considering the determination of metals in lubricating oil as

discussed below.

Determination of metals in lubricating oil

The determination of metals in lubricating oils used in aircraft, truck, locomotive and

other engines provide an excellent indication of the mechanical wear and tear of the

engine. Infact, as the concentration of certain metals increases, the wearing out partsor components of the engine can be identified resulting in a decision about their

replacement or repair. This routine programme of wear-metal analysis saves lot of

money all around the world. The most important wear metals that are monitored are

Fe, Al, Mg, Cu and Ag along with other trace metals. Iron appears as an indicator ofmore than 80 percent of all component failures detected by wear-metal analysis.

Aluminium usually relates to wear of oil pumps, cases, housings, pistons and cylinderheads, and copper to wear of bronze parts such as bushings and retainers. Silicon isuseful as an indicator of lubricant contamination from dust. These determinations are

generally made by spark AES method and the spectra of 10 or more elements in the

range of 0.1 to 500 ppm are determined.

SAQ 3

Enlist any five elements present in rock samples that are expressed in terms of their

oxide equivalents.

…………………………………………………………………………………………..

…………………………………………………………………………………………..

…………………………………………………………………………………………...

…………………………………………………………………………………………...

SAQ 4

What do you understand by wear metal analysis? What is its significance?

…………………………………………………………………………………………..

…………………………………………………………………………………………..

…………………………………………………………………………………………...

…………………………………………………………………………………………...

…………………………………………………………………………………………...

11.6 SUMMARY

Atomic absorption spectrophotometry (AAS) and atomic emission spectrometry

(AES) are being widely used for the elemental analysis of geological, biological,

environmental, industrial and other types of samples.

The salient features of AAS and AES along with a comparative account of the two

techniques, followed by the sample preparation for the analytical determination bythese technique have been recapitulated in this unit to give an overall picture of the

two techniques. The importance and principles of some important typical applications

of AAS and AES in diverse areas such as biological sample, environmental samples,and industrial samples have also been discussed.


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Atomic SpectroscopicMethods-II 11.7

TERMINAL QUESTIONS

1. Explain why simultaneous multielemental determination by ICP-AES is easiercompared to that by AAS.

2. Explain the difference between atomic emission and atomic absorption

spectrometry.

3. Define the following terms;

a) Plasma,

b) Spectral interference

4. Explain the observation during AAS determination of uranium where a linearrelationship is observed in the concentration range of 500 to 2000 ppm. At lower

concentrations, the relationship is nonlinear which, however, becomes linear if2000 ppm of alkali metal salt is added.

5. Explain the following observations:

a) Atomic emission is more sensitive to flame instability than atomicabsorption.

b)

Monochromators of higher resolution are found in ICP-AE spectrometersbut not in flame AA spectrometers.

c) Inductively coupled plasmas are suitable for atomic emission

spectrometry but it is rarely used for AAS.

6. Copper was determined in an aqueous sample by AAS following standardaddition method. First 10.0 mL each of the sample was pipetted into each of50.0 mL volumetric flasks. A standard containing 12.5 ppm of Cu were added to

the flasks and the solutions were made up to the volume. Following were theabsorbances.

Standard, mL Absorbance

0 0.201

10.0 0.292

20.0 0.378

30.0 0.467

40.0 0.554

Plot absorbance as a function of volume of the standard and calculate the

concentration of Cu in the sample.

11.8 ANSWERS

Self Assessment Questions1. In the process of preparing the sample for AAS or AES,

• all possible contamination coming from the air, from the skin of thesample collector, additives and reagents used in the analysis, as well as

parts of instrumentation including glass or plastic wares should be

avoided.


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Applications of AAS

and AES• biological materials of human and plant origin must be handled with

extreme care due to their inhomogeneity especially for trace element

analysis.

• body fluids such as blood, viscera, urine, etc should be stabilised and

homogenised so as to avoid occurrence of any changes in their

composition, prior to actual analysis.

•

the total number of transfers should be kept to a minimum.

2.

Some of the important applications of AAS in the area of environmentalanalysis are as follows

• Analysis of airborne particulate matter

•

Determination of mercury in air/water

• Determination of trace element contamination in Soil

3. The following elements are conventionally quoted as their oxide equivalents:

Si, Al, Fe, Mg, Ca, Na, K, Ti, Mn, and P

4.

The wear metal analysis refers to the determination of metals in used lubricating

oils from the aircraft, truck, locomotive and other engines. This provides an

assessment of the mechanical wear and tear of the engine.

Terminal Questions

1. In case of ICP-AES all the elements get excited at the same time in the plasmatorch. The radiation emitted by them can be measured sequentially or

simultaneously. Hence it is easier to determine several elementssimultaneously. However, in case of AAS a line source hollow cathode lamp is

used as the radiation source. As only one analyte element is able to absorb theradiation emitted by the cathode lamp we can measure only one element at a

time. For multielemental determination by AAS we need to use a cathode lamp

for each element thus making it a difficult task.

2. Basic difference between atomic emission (AES) and atomic absorption (AAS)

spectrometry is the source of radiation and the measured parameter. In AES, thesource of radiation is sample itself where the energy for excitation of analyteatoms is supplied by plasma, a flame, an oven or an electric arc or spark. The

signal is the measured intensity of the source at the wavelength of interest. On

the other hand in case of AAS, the source of radiation is a line source such ashollow cathode lamp. The signal is in terms of absorbance calculated from the

radiant power of the source and the resulting power after the radiation has

passed through the atomised sample.

3.

a) Plasma is a conducting gas that contains a large concentration of ionsand/or electrons.

b) Spectral interference is due to overlap of lines of an element in the

sample matrix with that of an analyte.

4. Deviations from linearity at low concentrations are often due to significant

ionization of the analyte. When an easily ionized element salt such as that ofalkali metal is added in excess amount then the ionization of analyte is

suppressed because of the electrons produced by ionization of the metal.

5. a) In AES, the analyte signal is produced by the small number of excitedatoms or ions whereas in AAS the signal is obtained from absorption by

much larger number of unexcited species. Any small change in flame


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Atomic SpectroscopicMethods-II conditions influence the number of excited species. Whereas such changes

have insignificant effect on the number of unexcited species.

b) Monochromator plays an important role in the resolution and selectivity of

ICP emission. Thus a high resolution monochromator can isolate theanalyte spectral line from other lines and background emission and reduce

spectral interferences. In AAS, however, resolution comes primarily fromspecific line emitted by a hollow cathode lamp. The monochromator

isolates only the emission line of the analyte element from lines of

impurities and fill the gas where a much lower resolution is needed.

c) Temperature of inductively coupled plasma is quite high which favours

the formation of atoms and ions. Also sample residence times are long so

that desolvation and vaporisation are complete. Further atoms and ions areformed in a nearly chemically inert atmosphere. Nearly constant electronconcentration leads to fewer ionization interferences. Since excited state is

not formed or it is less stable because of high temperature, it is not useful

for AAS.

6. Concentration of Cu as obtained from the plot is 28.0 ppm.


17/22


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Atomic SpectroscopicMethods-II INDEX

Absorbance 7, 8, 9, 14, 59, 63, 65 Acid digestion method 40, 64

Advantages and disadvantages of GFAAS 16Agricultural science 51

Analytical methodology in ICP-AES 47Qualitative analysis using ICP-AES 48

Characteristic line groupings 48Line coincidences 48Persistent or RU 48Spectral line tables 48

Quantitative analysis 48

Appearance of ICP plasma 37Applications of AAS 62

Biological samples 62Determination of calcium in serum 62Determination of cadmium 63

Determination of lead 63

Zinc in plant leaves 64Environmental samples 64

Analysis of airborne particulate matter 64Mercury in air/water 64

Trace element contamination in soil 65Industrial samples 65

Determination of molybdenum in steel 65

Tin in canned fruit juice 66 Applications of AES 66

Biological samples 67Determination of sodium in serum 67

Geological samples 67Environmental samples 68

Trace elements in airborne particulate matter 68

Industrial samples 69Determination of metals in lubricating oil 69

Applications of ICP-AES 51Agricultural science 51

Environmental science 51Forensic sciences 51Geological sciences 51

Health sciences 51Industry 51

Metallurgy 51

Applications of atomic absorption spectrophotometry 26Merits and limitations of atomic absorption spectrophotometry 27

Argon gas supply 36

Argon plasma spectroscopy 34

Atomic absorption spectrophotometers 17Double beam atomic absorption spectrophotometer 18

Single beam atomic absorption spectrophotometer 17

Atomic emission spectrometry based on plasma sources 33

Atomisers 11Auxiliary gas 35Background absorption 20

Biological samples 57, 61, 62, 67 Burners 12Calibration plot method 8

Carbon rod 14CCD based spectrometers 47

Characteristic line groupings 48


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Applications of AAS

and AESChemical interferences 20Chemical interferences 50Choice of argon as plasma gas 38

CID based instruments 47

Comparison between AAS and AES 59

Concentration dependence of absorption 7Continuum sources 10Dc electrical source 34

Detectors 13, 39Direct current plasma 37

Double beam atomic absorption spectrophotometer 18

Dry attack method 41Echelle spectrometers 46Electrodeless discharge lamps 11

Electrothermal atomisers 11, 14

Electrothermal vaporisation 42Environmental samples 64, 66, 68

Environmental science 51

Filament 14Flame atomiser 11

Forensic sciences 51

Frit nebuliser 42 Fuel-oxidant ratio 12Furnace atomic absorption spectrophotometry 14

Geological samples 67Geological sciences 51Graphite furnace 15, 58, 60

Graphite furnace atomic absorption spectrophotometry 14Advantages and disadvantages of GFAAS 16

Carbon rod 14

Electrothermal atomisers 14Graphite furnace 15

Filament 14

Furnace atomic absorption spectrophotometry 14Graphite tube 14

Handling background absorption in GFAAS 16L’Vov furnace 14

Graphite tube 14

Handling background absorption in GFAAS 16

Health sciences 51Hollow cathode lamp 11

Hydride generation 42

Hydride generation technique 24Industrial samples 59, 65, 69

Industry 51

Instrumentation for ICP-AES 39Detector 39Monochromator 39

Nebuliser 39Plasma source 39Processing and readout device 39

Sample Introduction 40Dry attack method 41 Electrothermal vaporisation 42Frit nebuliser 42 Hydride generation 42Nebulisation 41

Nebuliser 42Nebulisers for ICP-AES 42


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Atomic SpectroscopicMethods-II

Sample preparation 41Acid digestion method 41

Ultrasonic 42 Instrumentation for atomic absorption spectrophotometry 10

Atomisers 11Burners 12

Premix nebuliser-burner 12 Total consumption burner 12 Turbulent flow burner 12

Flame atomiser 11Fuel-oxidant ratio 12

Detectors 13Monochromators 13

Radiation sources 10Continuum sources 10Electrodeless discharge lamps 11Hollow cathode lamp 11

Line sources 10

Readout devices 14

Interferences in ICP-AES 50Chemical interferences 50Physical interferences 50Spectral interferences 50

Interferences in atomic absorption spectrophotometry 19Chemical interferences 20

Physical interferences 20Spectral interferences 19

Background absorption 20 Internal standard method 8L’vov furnace 14Lambert-beer’s law 7

Line coincidences 48

Line sources 10Mechanism of plasma formation 36

Merits and limitations of atomic absorption spectrophotometry 27Metallurgy 51

Microwave digestion 22Microwave digestion system 22

Microwave frequency generator 34Microwave induced plasma 38Monochromators 13, 39

Nebulisation 41Nebuliser 39, 42Nebulisers for ICP-AES 41

Persistent or RU 48

Physical interferences 20, 50

Plasma and its characteristics 34Argon plasma spectroscopy 35Choice of argon as plasma gas 38

DC electrical source 35

Direct current plasma 37Inductively coupled plasma 35

Appearance of ICP plasma 37Argon gas supply 36Auxiliary gas 35Mechanism of plasma formation 36Quartz tube 35Radio frequency power generators 36

Three electrodes DCP 37Torch 35Toroidal plasma 36Work coil 36

Microwave frequency generator 35


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Applications of AAS

and AES

Microwave induced plasma 38Plasma sources 34

Radio frequency generator 35

Plasma source 34, 39Polychromators 45

Premix nebuliser-burner 12Preparation of the sample 21

Principle of atomic absorption spectrophotometry 6Concentration dependence of absorption 7

Absorbance 7 Lambert-beer’s law 7

Quantitative methodology 7Calibration plot method 8Internal standard method 8Standard addition method 9

Principle of atomic emission spectrometry 32Atomic emission spectrometry based on plasma sources 33

Processing and readout device 39

Qualitative analysis using ICP-AES 48

Quantitative analysis 48Quantitative methodology 7

Quartz tube 35

Radiation sources 10

Radio frequency generator 35Radio frequency power generators 36

Readout devices 14

Rowland circle 47Sample handling in atomic absorption spectrophotometry 21

Microwave digestion 22Microwave digestion system 22

Preparation of the sample 21Sample introduction methods 23

Electrothermal vapourisation 24Hydride generation technique 24Ultrasonic nebulisation 24

Scrubbing 21Use of organic solvents 22

Salient Features of AAS 58Salient Features of AES 59Sample introduction 40Sample introduction methods 23

Sample preparation 40, 61

Scrubbing 21Sequential spectrometers 44

Simultaneous spectrometers 45

Single beam atomic absorption spectrophotometer 17Skew scan instruments 44

Solid state array detector spectrometers 46

Spectral interferences 19

Spectral interferences 46, 50

Spectral line tables 48Standard addition method 9Three electrodes DCP 37Torch 35

Toroidal plasma 36

Total consumption burner 12

Turbulent flow burner 12Types of instruments for ICP-AES 44

Sequential spectrometers 44


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Atomic SpectroscopicMethods-II

Skew scan instruments 44

Simultaneous spectrometers 45Polychromators 45

Echelle spectrometers 46

Solid state array detector spectrometers 46CCD based spectrometers 47

CID based instruments 47

Rowland circle 47

Ultrasonic 42 Ultrasonic nebulisation 24

Use of organic solvents 22Work coil 36

Documents

Unit 11 Applications of AAS and AES