unit 9 atomic absorption

8/3/2019 Unit 9 Atomic Absorption

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Atomic Absorption

SpectrophotometryUNIT 9 ATOMIC ABSORPTION

SPECTROPHOTOMETRY

Structure

9.1 IntroductionObjectives

9.2 Principle of Atomic Absorption SpectrophotometryConcentration Dependence of AbsorptionQuantitative Methodology

9.3 Instrumentation for Atomic Absorption SpectrophotometryRadiation SourcesAtomisers

Monochromators

Detectors

Readout Devices

9.4 Graphite Furnace Atomic Absorption SpectrophotometryElectrothermal Atomisers

Handling Background Absorption in GFAASAdvantages and Disadvantages of GFAAS

9.5 Atomic Absorption SpectrophotometersSingle Beam Atomic Absorption Spectrophotometer

Double Beam Atomic Absorption Spectrophotometer

9.6 Interferences in Atomic Absorption SpectrophotometrySpectral Interferences

Chemical InterferencesPhysical Interferences

9.7 Sample Handling in Atomic Absorption SpectrophotometryPreparation of the Sample

Use of Organic Solvents

Microwave DigestionSample Introduction Methods

9.8 Applications of Atomic Absorption Spectrophotometry9.9 Summary9.10 Terminal Questions9.11 Answers9.1 INTRODUCTIONYou have learnt in Block 3 that in atomic spectrometry, the elements present in a

sample are converted into gaseous atoms by a process called atomisation and their

interaction with the radiation is measured. In Units 7 and 8 of the third block youhave learnt about flame photometry and atomic fluorescence spectrometry. In flamephotometry we measure the emission of radiation by thermally excited atoms whereas

in atomic fluorescence spectrometry we monitor the fluorescence emission from the

radiationally excited atoms. In this unit you would learn about atomic absorptionspectrophotometry (AAS) that concerns the absorption of radiation by the atomised

analyte element in the ground state. The atomisation is achieved by the thermal

energy of the flame or electrothermally in an electrical furnace. The wavelength(s) ofthe radiation absorbed and the extent of the absorption form the basis of thequalitative and quantitative determinations respectively.

The atomic absorption methods using flame are rapid and precise and are applicable

to about 67 elements. Electrothermal methods of analysis on the other hand are slower

and less precise; however, these are more sensitive and need much smaller samples.

As the absorption of resonance radiation is highly selective and also very sensitive,the technique of AAS has became a powerful method of analysis, which is used for


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Atomic SpectroscopicMethods-II

trace elemental determinations in most analytical laboratories for a wide variety of

applications.

We begin the unit with an understanding of the origin of atomic absorption spectrum

and learn about the principle behind atomic absorption spectrophotometry being used

as an important analytical technique. Then we will take up the instrumentationrequired for the measurement of atomic absorption spectrum. An account of the

possible interferences in atomic absorption spectrophotometry will be followed by

sample handling procedures, like preparing and loading the sample for the spectral

measurements. The qualitative and quantitative applications of atomic absorptionspectrophotometry will be followed by the merits and demerits of the method. In the

next unit you would learn about atomic emission spectrometry and its applications indiverse areas.

Objectives

After studying this unit, you will be able to:

explain the principle of atomic absorption spectrophotometry, outline the quantitative methodology of the atomic absorption

spectrophotometry,

draw a schematic diagram illustrating different components of a flame atomicabsorption spectrophotometer,

justify the usage of line radiation sources in atomic absorptionspectrophotometry,

describe the functioning of different types of nebulisers used in atomicabsorption spectrophotometry,

compare the flame and flameless atomisation of the analyte in terms ofsensitivity and detection limits,

outline the importance of sample handling in atomic absorptionspectrophotometry,

discuss the interferences observed in atomic absorption spectrophotometricdeterminations, and

state the merits and limitations of the atomic absorption spectrophotometrictechnique.

9.2 PRINCIPLE OF ATOMIC ABSORPTIONSPECTROPHOTOMETRY

The concept of atomic absorption spectrometry (AAS) was proposed by two groups in

1955, A. Walsh of Australia and another one of C T J Alkamade and J M W Milatz

from The Netherlands. You have learnt that in atomic spectroscopy, the analyte must

be present in the atomic vapour state. In atomic absorption spectrophotometry theatomisation is performed by aspirating the sample solution into a flame where the

analyte element is converted into gaseous phase atoms. Alternatively, the sample isfed into a graphite furnace where the atomisation is achieved electrothermally atrelatively lower temperature, below 3000 K. As the temperature of atomisation is low;

most of the atoms remain in the ground state which can absorb characteristic radiation

from the radiation source made from the analyte element. The atomic vapourscontaining free atoms of an element in the ground state are illuminated by a radiation

source emitting the characteristic radiation of the analyte. You would recall fromUnit 8 that in halogen cathode lamp the cathode is made of the element that needs to

be determined and gives radiations characteristic of the element.

It may be noted that as

only ground state atomsare involved in this

process. Therefore, the

ionisation occurring due

to high temperature of theflame needs to be kept toa minimum.


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Atomic Absorption

SpectrophotometryThe radiation is absorbed by the analyte vapours and its intensity decreases. This is

similar to spectrophotometry which you have learnt earlier in Unit 2; molecules beingreplaced by atoms and the lamp changed to a line source. The degree of absorption is

a quantitative measure of the concentration of ground state atoms in the vapours. The

analysis is done by comparing the observed absorption with the one obtained bysuitable standard samples of the analyte under similar experimental conditions, i.e., a

calibration curve method is generally employed.

9.2.1 Concentration Dependence of AbsorptionYou have learnt earlier that according to Boltzmann distribution law, the population

of the ground state i.e., the number of species in the ground state is highest and it

keeps on decreasing as we go to higher energy levels. It can be shown that for mostelements at moderate temperatures prevailing in a flame, nearly all atoms are in

ground state leaving only a few atoms in excited state (Refer Example 1, Unit 7, page8, Block 3). The absorption follows Lambert-Beers law so that the concentration of

an analyte element in the vapours in the flame may be determined.

You would recall from Unit 2 that according to Lambert-Beers law, the extent of

radiation absorbed by the absorbing species is a function of the path length and theconcentration of the absorbing species.

Mathematically, bcP

P=0log

where,Po = radiant power of incident light,

P = radiant power of transmitted light,b = thickness of the absorbing medium,

= absorption coefficient, and

c = concentration of absorbing analyte atoms.

The term, log Po/P is called absorbance and is represented as A. Therefore we can

write it as follows.

bcAP

P

==

0

log

Thus, absorbance of the sample is directly proportional to the concentration of theanalyte. Therefore, a calibration plot of concentration of analyte element versus

absorbance is drawn from the standard solutions and the concentration of element in

unknown solution is read directly from the graph. However, such a linear relationship

between the absorption and the concentration can be observed only if all radiationpassing through the sample is absorbed to the same extent by the analyte atoms.However, the experimental concentration versus intensity calibration curve is

observed to be deviating from the linearity as a result of the presence of nonabsorbedradiation and other interferences. Therefore, suitable measures need to be taken so as

to minimise the interferences and obtain the linearity in the calibration curves. We

would discuss about these interferences in Section 9.6. Let us learn about the

methodology used in quantitative determinations using atomic absorptionspectrophotometry.

9.2.2 Quantitative MethodologyLike many other analytical methods, AAS is also not an absolute method of analysis.

The routine analytical methodology for quantitative determinations using AAS is

based on calibration method. Besides this, the internal standard method and standard

addition methods are also employed. You have learnt about these methods in Unit 7in the context of flame photometry. These are briefly recalled here.

Typical absorbance must

be in the range 0.1 to 0.3or else precision is poorer

at the extremes due to

instrumental noise.

In AAS the absorption of

resonant radiation byground state atoms of the

analyte is used as the

analytical signal.


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Atomic SpectroscopicMethods-II Calibration plot method

In this method, a calibration plot is drawn by aspirating standard solutions of knownconcentration into the flame and measuring absorbance for each solution. The

concentration of the unknown solution is then determined from the calibration plot.

Despite the fact that Beers law is followed in AAS, in practice the departures fromlinearity are encountered as shown in Fig. 9.1. The nonlinearity is due to the

transmission of unabsorbed light from the radiation source. In addition, a number of

uncontrolled variables in atomisation and absorbance measurements may also affect

the measurements. Therefore, we need to find the concentration range in which theLambert-Beers law holds i.e., we get a straight line.

Fig. 9.1: Typical calibration plot between the absorbance and the concentration of

analyte element

In practice, however, a single calibration does not serve the purpose. We need to take3-4 standards of different concentration and a bank to obtain a suitable calibrationplot. As you can see in Fig. 9.1, a single standard calibration plot does not hold good.

Further, if the analyte concentration happens to be outside the limits of the standards

used for calibration then the analyte sample should be suitably diluted orconcentrated.

As the test solution is often a complex whose all constituents are not known, it

becomes almost impossible to prepare standard solutions having a similarcomposition to the analyte sample to obtain a calibration plot. In such cases we have

to use internal standard method and the standard addition method.

Internal standard method

In this method, a fixed amount of an internal standard which is chemically similar tothe analyte being determined and absorbs at similar wavelength, is added to the

standard solution and the test sample. The intensity ratio of the analyte and internalstandard is plotted as a function of the analyte concentration in the standard solution.For example, while determining Na or K in blood serum Li is used as internal

standard. A typical plot obtained in an atomic absorption spectrophotometric

determination using internal standard method is given in Fig. 9.2.


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Atomic Absorption

Spectrophotometry

Fig. 9.2: A typical calibration plot for internal standard method in AAS

If the aspiration fluctuates then each signal is affected to the same extent and the ratio

at a given analyte concentration remains constant.

Standard addition method

As you have learnt in Unit 7, this method is especially applicable when the signal is

altered by the sample matrix. In this method a known amount of the standard solutionof known increasing concentrations of the analyte is added to a number of aliquots of

the sample solution. The resulting solutions are diluted to the same final volume and

their absorbances are measured. A graph is drawn between the absorbance and theadded concentrations of the analyte. It is then extrapolated to the concentration axis toobtain the concentration of sample solution. If the plot is nonlinear then extrapolation

is not possible. It is essential to perform blank correction in such a case. The

calibration plot obtained by using standard addition method is shown in Fig. 9.3.

Fig. 9.3: Calibration plot for standard addition method indicating Cx as the

concentration for unknown sample

AAS is a promising analytical method that is extensively employed for quantitative

determinations of different elements in wide range of samples. A major disadvantage

of the AAS measurements is that only a single element can be determined at a time asa separate radiation source is required for each element. However, nowadays modern

instruments are equipped to undertake multielement determinations. Let us learn

about the instrumental aspects of atomic absorption spectrometry.

SAQ 1

What is the importance of calibration plot in atomic absorption spectrophotometry?

..

..

..


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Atomic SpectroscopicMethods-II 9.3 INSTRUMENTATION FOR ATOMIC ABSORPTION

SPECTROPHOTOMETRY

You have learnt above that in AAS the absorption of resonant radiation by groundstate atoms of the analyte is used as the analytical signal. Accordingly, a sourcedelivering the characteristic resonant radiation of the analyte is required along with an

atom reservoir into which the analyte is introduced and atomised. The basic

requirements of atomic absorption spectrophotometer instrument are similar to any

regular spectrophotometer; the sample holder cell being replaced by a flame or someother atomiser. A typical atomic absorption spectrophotometer consists of the

following components.

Radiation source Atom reservoir Monochromator Detector Readout deviceA block diagram showing the basic components of an atomic absorptionspectrophotometer is given in Fig. 9.4.

Fig. 9.4: Schematic diagram of an atomic absorption spectrophotometer showing its

basic componentsIn a typical flame atomic absorption spectrophotometric determination, the radiationfrom a hollow cathode lamp is made to fall on the sample of the analyte aspirated into

the flame, where a part of it is absorbed. The transmitted radiation is then dispersed

by a monochromator and sent to the detector. The detector output is suitablyprocessed and is displayed by appropriate readout device. These single channel

instruments can perform measurements at a single wavelength only in one channel.

Nowadays, dual-channel instruments are also available that permit simultaneousmeasurements at two different wavelengths. These contain two independentmonochromators for the purpose. Thus, these can be used for the simultaneous

determination of two elements; one can be the analyte to be determined and the other

may be a reference element. You would learn about different types of AASinstruments in Section 9.5.

Let us learn about various components of an atomic absorption spectrophotometer.

9.3.1 Radiation SourcesAll commercially available atomic absorption spectrophotometers use a radiation

source that emits the characteristic spectrum of the element to be determined. Theessential requirement of the radiation source is that it gives a constant and intense

output. Generally two types of sources are in use: line sources and continuum

sources. Initially continuum sources were used and the primary radiation required


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Atomic Absorption

Spectrophotometrywas isolated with a high resolution monochromator. However, these had low radiant

densities and did not provide sufficiently high sensitivity. Nowadays, the hollow-cathode lamp (HCL), belonging to the first type, has been most widely used. The

electrodeless discharge lamps(EDL) - another line sourcesare also frequently

employed for the purpose and in fact are superior for the elements such as As, Se andTe with low melting points. You have learnt about HCL and EDL in the context of

atomic fluorescence spectrometry in Unit 8.

When the bandwidth of the primary radiation is low with respect to the profile of the

analyte absorption, a given amount of analyte would absorb more radiation.Therefore, the radiation sources having low widths of the emitted analyte lines arepreferred. Accordingly, the radiation sources are designed so as to operate at muchlower temperatures and pressures as compared to that of the flame and furnaces used

for atomisation. As a consequence, the emitted lines are much sharper than the

absorption lines to be measured. In such a set up, sufficient accurate measurements of

the peak absorption can be made without using elaborate optics. It is referred to assource resolution. Fig. 9.5 illustrates source resolution achieved by using radiationsources emitting sharper lines.

Fig. 9.5: A schematic diagram illustrating the source resolution achieved by usingradiation sources emitting sharper lines

9.3.2 AtomisersThe purpose of atomiser is to provide a representative portion of the analyte in theoptical path and convert it into free neutral ground state atoms. In atomic absorptionspectrophotometry, the flames and furnaces that generate a temperature in the range

of 1500 to 3000 C are the most common methods of atomisation. Two common types

of atomisers used for generating atomic species in the vapour phase are flame

atomisers and electrothermal atomisers. Let us learn about flame atomisers. You willlearn about electrothermal alone use in flameless atomic absorption spectrum.

Flame atomiser

You have learnt about flame atomisers in Unit 7. In a typical flame atomisationprocess, the analyte solutions are generally nebulised with the help of a nebuliser (see

Sec. 7, Unit 7) into a spray chamber. The aerosol so produced along with a mixture ofa burning gas and an oxidant is directed into a suitable burner. As already described inUnit 7, Section 7.5, flame temperature depends on fuel-oxidant ratio and the requisite

temperature for analysis can be obtained by varying the fuel-oxidant ratio. The fuel-

oxidant combinations commonly used in AAS, the corresponding combustion

reactions and the flame temperatures are given in Table 9.1.

The availability of narrow

band and tunable laser

sources have opened upnewer areas of application

of the technique.


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Atomic SpectroscopicMethods-II Table 9.1: The fuel-oxidant combinations commonly used in AAS, the

corresponding chemical reaction and the corresponding flame

temperatures

Fuel-oxidant Combustion reaction* Flame

temperature

(K)

C3H8 air C3H8+5O2+20N2 3CO2+4H2O+20N2 2267

H2 air 2H2+O2+4N2 2H2O+4N2 2380

C2H2 air C2H2+O2+4N2 2CO+H2+4N2 2540

H2 O2 2H2+O2 2H2O 3080

C3H8 O2 C3H8+5O2 3CO2+4H2O 3094

C2H2 N2O C2H2+5N2O 2CO2+H2O+5N2 3150

C2H2 O2 C2H2+2O2 2CO2+H2 3342

*N2 is included in air just to show its stoichiometry.

While analysing liquid samples, flame is considered to be superior in terms of

performance characteristics and reproducible behaviour though sampling efficiency

and sensitivity of other methods are better. This is because large amount of the

sample flows down the drain and the residence time of individual atoms in the pathlength of flame is of the order of ~0.1ms. The region of maximum absorption is

restricted to specific areas of the flame.

Concentration of atoms may vary widely if the flame is moved relative to the light

path either vertically or laterally from the resonance line source. The position of

observation in the flame and the fuel-oxidant ratio must be suitably optimised foreach element in AAS. The fuel-oxidant ratio and observation heights are so chosen asto provide the maximum number of free atoms while minimising interferences from

emission, ionisation or compound formation.

Burners

Two major types of nebuliser burners used in AAS are premix nebuliser-burner

system and total consumption burner. You have learnt about these in the context of

flame photometry in Unit 7. You would recall that in premix type burner, liquid issprayed into a mixing chamber where the droplets are mixed with the combustion gas

and are sent to the burner. Fig. 9.6 gives a schematic drawing of such a burner used inAAS.

On the other hand, in the total consumption burner, the nebuliser and burner are

combined. This is also called turbulent flow burner (Fig. 7.1). Several factors are

involved in the choice of a burner. Generally speaking, a premix burner is preferredfor atomic absorption work, except when a high burning-velocity flame must be used.Turbulent flow burners are widely used for atomic emission measurements about

which you would learn in the next unit.

The flame atomisation method discussed above is used more often than the others. As

many as 67 elements can be determined by employing simple, easy to use,


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Atomic Absorption

Spectrophotometryequipments based on flame atomisation. It is facilitated by the usage of sharp line

light sources that result in the ease and reliability in selection of resonance lines.More so, the method is rapid and gives a precision of as high as 0.1 % in some

cases.

Fig. 9.6: Schematic diagram of premix nebuliser burner system used in AAS

Flame atomisation, however, has following disadvantages.

i) Only about 10% of the nebulised sample reaches the flame and it is then furtherdiluted by the fuel and oxidant gases so that test material has very smallconcentration in the flame.

ii) A minimum sample volume of 0.5-1.0 mL is needed to give a reliablemeasurement.

iii) Viscous samples such as blood, serum, oils etc require dilution with a solvent.In order to avoid such problems, nonflame methods involving electrical heating havebeen developed for atomisation about which you would learn in the next section.

9.3.3 MonochromatorsYou know that the monochromators are the devices that can selectively provideradiation of a desired wavelength out of the range of wavelengths emitted by the

source or emitted by analyte sample. In AAS, the monochromators select a given

emission line and isolate it from other lines due to molecular band emissions and allnon absorbed lines. Some of these lines originate from the filler gas in the hollow

cathode lamp while some others are the spectral emissions of various samplecomponents during atomisation. Most commercial AAS instruments use diffractiongratings as monochromators.

9.3.4 DetectorsAs the wavelengths of resonance lines fall in UV region, the most commonly useddetector in atomic absorption spectrophotometry is photomultiplier (PM) tube whose

output is fed to a readout system. The radiation received by the detector may originate

The flames are not idealatomisers as for a number

of elements theatomisation is not

quantitative. The

sensitivity is loweredconsiderably due to this

and by dilution of theanalyte atom population

with gases in the flame.


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not only from the selected resonance lines but also from the emission within the

flame. Therefore, in addition to absorption signal intensityIA, the detector mayreceive signal intensity of (IA + S) where S is the intensity of emitted radiation from

flame. Actually one requires only the signal intensity due to absorption, it is therefore,

important to eliminate effects due to flame emission. This is achieved by modulatingthe emission from the emission line source using a mechanical chopper device.

9.3.5 Readout DevicesThe readout systems include meters, chart recorders and digital display meters. Thesedays, however, microprocessor controlled systems are commercially available where

everything can be done by touch of a button. Modern instruments provide a fast

display of the experimental conditions, absorbance data, statistical values andcalibration curves, etc.

SAQ 2

Name the line sources employed in atomic absorption spectrophotometry.

..

..

..

..

..

..

9.4 GRAPHITE FURNACE ATOMIC ABSORPTIONSPECTROPHOTOMETRY

As mentioned earlier, the flame atomisation method suffers from some drawbacks. Inorder to overcome these problems some flameless methods of AAS have been

developed. Two types of flameless atomisers are generally used. These are graphite

tube or LVov furnace and the carbon rod or filament. AAS using graphite furnaceis called GraphiteFurnace Atomic Absorption Spectrophotometry (GFAAS)

which is highly sensitive (100 to 1000 times as compared to flame AAS) and requiresa very small sample size. It has a further advantage of not requiring any samplepreparation. So much so that solid samples do not require sample dissolution. Let us

learn about the electrothermal or flameless atomic absorption spectrophotometry.

The basic principle of flameless AAS is similar to flame AAS. The analyte isconverted into vaporised atoms in ground state that are subjected to the characteristicresonance radiation emitted by a line source. The absorption of radiation and its

extent form the basis of analytical applications. As regards the instrumental aspects,the two techniques are similar to a good extent, the difference being in the atomiser

and the atom reservoir. Rest of the components of the instrument are the same,

however, a faster electronics is required to process the rapidly obtained transientsignal in GFAAS. Let us learn about the electrodeless or electrothermal atomisers.GFAAS is also termed as electrothermal AAS because of the electrothermal atomisers

used.

9.4.1 Electrothermal AtomisersThe use of furnaces as atomisers for quantitative AAS goes back to the work of

LVov, which led to the breakthrough of atomic absorption spectrophotometry

towards very low absolute detection limits. The essence of LVov method is to


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Atomic Absorption

Spectrophotometrycompletely vaporise a small amount of analyte sample in a graphite tube furnace and

obtain a good concentration of the analyte species in the vapour phase which can besuitably determined. In electrothermal AAS, graphite or metallic tube or cup furnaces

undergo resistive heating to attain temperatures required for complete atomisation of

the analyte. For volatile elements this can be accomplished at temperatures of 1000 Kwhereas for more refractory elements the temperatures should be up to 3000 K. Let us

learn about the working of graphite furnace as an atomiser.

Graphite furnace

A graphite furnace consists of a hollow graphite cylinder having a length of about5 cm and diameter of about 1 cm. The tube is surrounded by a metal jacket through

which water is circulated and remains separated from the tube by a gas space where

an inert gas such as argon or nitrogen is circulated as schematically shown in Fig. 9.7.A small amount of analyte sample solution (1-100 L) is introduced in the sample

cell or holder by inserting the tip of micropipette through a port in the outer jacket,and into the gas inlet orifice in the centre of the graphite tube. Alternatively the

powered analyte sample (about 10-500 g) is introduced directly into the graphitetube.

Fig. 9.7: Schematic diagram of the cross section of an electrically heated graphite

furnaceThe nature and design of cuvettes or sample holder is of great importance in GFAAS.

Different types of cuvettes are commercially available. The standard cuvette madefrom electrographite is suitable for the determination of volatile elements such as Pband Cd. Extended lifetime cuvettes can sustain faster heating rate and have longer

lifetime and are especially useful in the determination of refractory elements.

The graphite tube is heated by the passage of an electric current to a temperaturecapable of evapourating the solvent from the solution. The current is then increased in

such a way that initially the sample is ashed and then ultimately it is vaporised

producing metal atoms. In other words, a heating cycle as shown in Fig. 9.8(a) isfollowed. For reproducibility, the temperatures and the time of the drying, ashing and

atomisation process are carefully selected depending on the metal to be analyzed.

The radiation from the line source is passed through the central hollow graphite tubecontaining the vaporised analyte. The absorption signal produced by this method is a

transient one and lasts for a few seconds Fig. 9.8(b). The signal will be obtained onlywhen the analyte is atomised. You may note the correspondence between theatomization step and the analyte signal. This can be recorded on a suitable chart

recorder. This is in contrast with the flame atomisation technique wherein a steady

absorption signal is obtained. Each graphite tube can be used for 100-200 analysesdepending upon the nature of material.


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(a) (b)

Fig. 9.8: (a) A schematic heating cycle profile for graphite furnace (b) the transient

absorption signal obtained by GAAFS

The sensitivity of GFAAS is much higher as compared to the flame AAS and the

detection limits are lower by 2-3 orders of magnitude than that in flame AAS. This isso because in the furnace a much higher concentration of atomic vapour can be

maintained as compared with flames. Furthermore, in this method, the dilution of theanalyte by the solvent is avoided as the solvent is evapourated before the atomisation

step.

9.4.2 Handling Background Absorption in GFAASHigh background absorption is a problem area in furnaces. This may be solved by

diluting the sample or selecting another resonance wavelength line. The use of matrix

modifier is a commonly acceptable method used to reduce background effects. In thismethod a reagent is added to the sample that may modify the matrix behaviour and

thereby tackle the problem of background. Sometimes the added matrix may modify

the analyte also. Following are the reasons for adding matrix modifier.

It stabilises the analyte during the ashing stage. It converts the interfering matrix into a volatile compound that may be removed

during ashing.

It helps to obtain isothermal conditions in the graphite tube by delaying theanalyte atomisation.

9.4.3 Advantages and Disadvantages of GFAASThe most significant advantages of this flameless vaporisation method are:

It eliminates the possibility of the interaction of the sample with differentcomponents of the flame, thereby eliminating anomalous results.

The longer residence time for the analyte in the path of incident radiation leadsto a greater sensitivity.

As a higher proportion of the analyte sample is converted into vapours, thesensitivity is further enhanced.

It provides an ability to deal with very small sample sizes. This becomes quiteimportant in the context of clinical samples.

However, it has the following disadvantages too.

The background absorption effects are more serious. Analyte may be lost during ashing especially for the volatile compounds. The sample may not be completely atomised and it may produce memory

effect within the furnace.

Memory effect refers tothe contributions from the

remains of the previousdeterminations.

While using GFAAS,

great care is requiredduring sample preparation

because of contaminationarising out from

glassware and volumetricpipettes.


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Atomic Absorption

Spectrophotometry Precision is poorer than in flame AAS. However, furnace auto samplers have

enhanced the precision of furnace AAS.

Problems due to interferences and high background may become serious.Before proceeding further try to answer the following SAQ.

SAQ 3

What do you understand by heating cycle in the context of graphite furnace?

..

..

..

.

9.5 ATOMIC ABSORPTION SPECTROPHOTOMETERHaving learnt about the essential components of an atomic absorptionspectrophotometer let us now learn about the different types of atomic absorption

spectrophotometers. You would recall the spectrophotometers used in UV-VISspectrophotometry. It is pertinent to mention here that the atomic and molecular

absorption spectrometries are quite similar in principle and instrumentation; theradiation source and the sample holder being different. Let us learn about the two

types of atomic absorption spectrophotometers given below.

Single beam atomic absorption spectrophotometer Double beam atomic absorption spectrophotometer9.5.1 Single Beam Atomic Absorption SpectrophotometerA simplified sketch of a single beam flame atomic absorption spectrophotometer is

shown in Fig. 9.9. It consists of hollow cathode lamp (HCL), a radiation source, flame

as an atomisation device, a monochromator, a photomultiplier detector and a

recording system. The HCL radiation is chopped to eliminate the background signalarising from the radiation emitted by the sample itself, focused on the atomic vapour

produced by the atomiser and then directed to the monochromator where the atomicline of interest is isolated.

Fig. 9.9: Schematic diagram of a single beam flame atomic absorption

spectrophotometer


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The electronic amplifier is synchronised with the chopper so that the signal

component generated by emission from the sample is not detected. The attenuation ofthe source radiation by the analyte atomic vapour is detected by the photomultiplier.

A blank is aspirated into the flame and the transmittance is adjusted to 100%.

9.5.2 Double Beam Atomic Absorption SpectrophotometerIn this instrument the beam from the HCL is split by a mirrored chopper, one halfpassing through the atomiser and the other half around it, as schematically shown in

Fig. 9.10. The two beams are then recombined by a half-silvered mirror and passedthrough the monochromator. The ratio between the reference and sample signal is

then amplified and fed to the readout display and recorder. These instruments correct

the fluctuations in the intensity of radiation coming from the radiation source and forchanges in the sensitivity of the detector. It must be noted that reference beam in

double beam instruments does not pass through the flame and thus corrects for theloss of radiant power due to absorption or scattering by the flame itself.

Fig. 9.10: Schematic diagram of a double beam atomic absorption spectrophotometer

Changes in peak shape or position can be indicative of interference problems. Mostinstruments are equipped with a device for background correction. These days

simultaneous multielemental atomic absorption spectrophotometers with line source

for 2 to 10 elements have become available. The most successful design of asimultaneous multielement AAS is based on continuum source and a multichannel

direct reading spectrophotometer.

Modern atomic absorption spectrophotometers generally have the following features:

These have a lamp turret capable of holding at least four hollow cathode lampsemitting the absorption lines for different elements. These have an independent

current stabilised supply for each element.

The sample area is capable of incorporating an autosampler which can workwith both flame and furnace atomisers. Improved analytical precision isobtained when an autosampler is used in conjunction with a furnace atomiser.

The monochromator is capable of high resolution typically 0.04 nm, a featuremore desirable if the AAS is adapted for flame emission work though good

resolution is also desirable for many elements in AAS.

The photomultipliers are able to function over a wide wavelength range of180 800 nm.

The instruments are equipped with a background correction facility.


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Atomic Absorption

Spectrophotometry The instruments have an integral video screen facility for the ease of operation.

A modern software package includes help facilities, full graphical data

presentation, complete data storage and flexible data generation. The softwaremust optimise flame and spectrometer parameters including furnacetemperature.

SAQ 4

In what way is a double beam atomic absorption spectrophotometer better than a

single beam spectrophotometer?

..

..

..

..

9.6 INTERFERENCES IN ATOMIC ABSORPTIONSPECTROPHOTOMETRY

All spectrometric methods have associated interferences that need to be addressed to

so as to put the technique to analytical use. We may define an interference to be achemical, physical or spectral effect that may cause the analyte signal to be altered inintensity. Accordingly, there are three types of interferences in AAS. These arespectral, chemical and physical interferences. Let us learn about these in the contexts

of flame AAS and GFAAS.

9.6.1 Spectral InterferencesThese refer to the presence of another atomic absorption line or a molecular

absorption band close to the spectral line of the analyte element being monitored. Itqualifies to be an interference if it is not resolved by the monochromator. Most

probable spectral interferences are the ones of the molecular emissions from oxides ofother elements in the sample. In case of AAS, such interferences occur if a dc

instrument is used and can be eliminated by employing an ac instrument. Similarly apositive interference may occur if an element or molecule is capable of absorbing

radiation from a continuous source. This may be minimised but not eliminated

altogether by using a line source.

Another source of spectral interference is the light scattering or absorption by solid

particles, unvaporised solvent droplets or molecular species in the flame. This

problem is significant at wavelengths less than 300 nm when solutions of high saltcontent are aspirated. This arises because of incomplete desolvation and is calledbackground absorption or blank. This can be corrected by measuring the absorbance

of a line close to the absorption line of the analyte element but not absorbed by theelement itself. The measurements should be made at two other lines from the hollowcathode lamp or a nearby line from a second hollow cathode lamp. Analyte test

element should always be aspirated to check that it does not absorb the backgroundcorrection line. This technique requires two separate measurements on the sample.

A yet another source of spectral interference is the background emission from theflame. This may be corrected by modulation of the output of the radiation source andthe ac detection system. Several background correction schemes have been developed

and incorporated in spectrophotometers such as the deuterium background correction,

Zeeman correction system and the Smith-Hieftie system. These are not discussedhere, you can obtain information about these from the reference texts listed at the end

of the block.

Absorption due to

molecular species andscattering are more

problematic withelectrothermalatomization.


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Atomic SpectroscopicMethods-II 9.6.2 Chemical Interferences

These include interferences due to ionisation, formation of low volatility compounds,

dissociation, etc. During atomisation in the flame, several reactions occur resulting in

the formation of analyte compounds which decrease atomic population in the cell.

Most important chemical interference is due to anions and form atom of compoundsof low volatility from the analyte element. For example, the refractory elements suchas Ti, W, U, V, Mo, Zr and elements like B, Al, and Fe may combine with O and OH

species in the flame producing thermally stable oxides and hydroxides. Similarly,

absorbance due to Ca is decreased in presence of phosphate because of the formationof calcium phosphate having low volatility.

Such interferences can be avoided by increasing the flame temperature whence these

interfering compounds are decomposed. In some cases chemical interferences may beeliminated by using a releasing agent that react with interfering species and avoids its

reaction with the analyte element. For example, in the determination of Ca, Sr and Lacan be used as releasing agents to minimise phosphate interference as these would

react preferentially with the phosphate.

9.6.3 Physical InterferencesThese are independent of the analyte type and have nearly the same effect on

emission, absorption and fluorescence with a given type of atomiser. These may bedue to variations in the gas flow rates, changes in the solution viscosity affecting itsrate of aspiration into the flame which may finally change the atomic concentration in

the flame. Viscosity of standards and samples may be different if the sample contains

organic solvent or a high concentration of the salt which is not the case with thestandard. Errors due to changes in viscosity may be avoided by matching the matrix

and by performing frequent calibrations. Some instruments offer the capability ofusing internal standards that can partially compensate for changes in physical

parameters including flame temperature.

SAQ 5

a) How does phosphate interfere in the quantitative determination of calcium byatomic absorption spectrophotometry?

..

..

..

..

b) How is this interference handled?..

..

..

..

9.7 SAMPLE HANDLING IN ATOMIC ABSORPTIONSPECTROPHOTOMETRY

In principle, the sample in solid, liquid or in the gas phase can be analysed by flameAAS. However, in most cases, sample analysed by AAS is in the solution form.Therefore, the solid sample is first dissolved and converted into a solution. Solids


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Atomic Absorption

Spectrophotometrycould be analysed directly also by using an electrothermal furnace. The gaseous

samples, on the other hand, generally are pretreated by scrubbing before the resultantsolution. Alternatively, the gases may be adsorbed on a solid surface and then leached

into solution with suitable reagents. Let us learn how the dissolution of the solid

sample, an important step in AAS, is carried out.

9.7.1 Preparation of the SampleThe choice of proper reagents and techniques of decomposition and dissolution of the

sample is a critical step for the success of AAS determination. This is often done byacid digestion, which produces a clear solution without loss of any of the elements to

be determined. It is therefore, essential that all the reagents and solvents used in wet

decomposition should be of highest purity as any impurity may raise the blank value.Common acids used for dissolution are HCl, HNO3, aqua regia (HCl : HNO3 :: 3:1) or

perchloric acid (HClO4) which dissolve most of the inorganic materials. For thedecomposition of silicate materials, however, HF must be used. A combination of

nitric acid and perchloric acid is especially useful for the complete destruction of fatsand proteins in biological samples.

In a typical dissolution step, a suspension of the sample in acid is heated by flame or ahot plate until complete dissolution i.e., when the entire solid has disappeared and atransparent solution is obtained. The decomposition temperature is the boiling point

of acid. However, such a wet decomposition in open vessels may give rise tosystematic errors due to volatilisation losses and contamination caused by thereagents and container material, and loss of elements caused by adsorption on the

vessel surface. More so, sometimes the dissolution may not be complete and it may

also cause errors. In order to avoid such errors, wet decomposition methods in closedsystems have been developed. These have the following advantages.

There are no volatilisation losses. These have a shorter reaction time and improved decomposition due to high

temperatures.

The blank values are low. These do not have contamination from external sources.If the concentration of the elements to be determined is too high, then the solution

must be diluted quantitatively before commencing the absorbance measurements.

Conversely, if the concentration of the metal in the test solution is too low, aconcentration procedure such as solvent extraction or ion-exchange must be followed.

While analysing halogens and some other elements like S, Se, P, B, Hg, As and Sb, it

is advantageous to use combustion in an oxygen flask. The combustion is carried outin a sealed container and the reaction products are absorbed in a suitable solvent.

Metals and alloys can usually be dissolved in acids, whereas dissolution of glass

requires alkaline or acid fusion. Generally speaking, the final solution of the analyte

should not contain acid concentration more than about 1Mor else aspiration ofcorrosive solution may damage the burner.

9.7.2 Use of Organic SolventsIn the early development stages of AAS, it was observed that analyte solutionscontaining organic solvents of low molar mass e.g. alcohols, ethers, ketones and

esters enhanced absorption peaks. This has been attributed to the increased rate of

aspiration, nebulisation efficiency, formation of finer droplets, and more efficientevaporation or combustion of the solvent. In favourable cases, up to three fold

increase in sensitivity could be obtained by adding a miscible organic solvent such as

Sample preparation is a

crucial step in the AASdetermination.


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acetone to the solution. However, this makes the sample solution more dilute which

more or less defeats the purpose of achieving enhanced sensitivity.

Therefore, to obtain increased sensitivity, the technique of solvent extraction is

usually employed. The metal extracted into the organic phase is directly aspirated intothe flame. This method has following advantages.

The analyte element is separated from the bulk matrix of the sample therebyeliminating chemical interferences.

Complexation of metal with organic solvent increases atomisation efficiencycausing upto ten fold signal enhancement.

The analyte element may be extracted into a smaller volume of organic solventwith 10

to 100 fold gain in concentration. Methylisobutyl ketone (MIBK) is an

ideal solvent which is easily aspirated into the flame.

While using an organic solvent, flame should be adjusted before aspirating the solventwhich must be burned along with the fuel. If the flame is too rich in fuel, the solventwill not be burnt resulting in smoky flame. Thus fuel-oxidant ratio may be adjusted

while using organic solvent to offset the presence of organic solvent. Solvent should

be aspirated between samples because the hot lean flame will heat up the burner.However, the lean mixture results in lower flame temperature, thus increasing the

possibility of chemical interferences. Therefore, suitable safety procedure must befollowed while using organic solvents.

9.7.3 Microwave DigestionIn another method of preparing sample for AAS determination, microwave radiations

are employed. A microwave digestion system (MDS) offers more rapid and efficient

decomposition of complex matrices of geological and biological samples. Theconcept of microwave ovens for the decomposition of inorganic and organic samples

was first proposed during mid 1970s. This method has an advantage overconventional methods as it takes less time because of rapid heating ability ofmicrowaves.

In contrast to conventional flame/hot plate heating method based on conduction,microwave energy is directly transferred to all the molecules of solution almost

simultaneously without heating the vessel and thus boiling temperature is reachedvery quickly due to increased pressure in the vessel. In addition, small amounts of

reagents are used and evaporative losses are avoided, thus reducing interferences byreagent contamination. As MDS can be easily automated it greatly reduces the

operator time to prepare samples for analysis. These days multi-vessel MDS with 4, 6

or 8 vessels are commercially available where more number of samples can besimultaneously dissolved.

Microwave digestion vessels are constructed from low-loss materials that aretransparent to microwave radiation. Teflon is an ideal material for many of the acids

including HF commonly used for dissolution. Not only it is transparent to microwaves

but it has low melting point of ~300 oC which is of course lower than boiling point ofH2SO4 and H3PO4. For these acids, quartz and borosilicate glass vessels are used.A typical closed vessel microwave digestion system consisting of teflon body, capand a safety relief valve is shown in Fig. 9.11. The maximum recommended

temperature obtained with this device is 250 C. When overpressurisation occurs,safety valve gets distorted similar to home pressure cookers and the excess pressure is

released.

Microwave region ofelectromagnetic spectrum

corresponds to lower

energy (or longer

wavelength) than IRradiation. Most

commonly used

frequency is 2450 MHz asset by international

convention for use forindustrial, scientific and

medical purposes.


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Atomic Absorption

Spectrophotometry

Fig. 9.11: A schematic diagram of closed vessel microwave digestion system

Commercially available MDS incorporates corrosion protection for the interior with

variables such as sample mass (0.1-2g), digestion acids (HCl, HF, HNO3 and H3BO4),

power setting and heating time (1-20min), etc.

9.7.4 Sample Introduction MethodsThe sample introduction into the flame is an important step in flame AASmeasurements as its accuracy, precision and detection limits depend on how the

analyte sample is introduced. The aim of a sample introduction system is to transfer a

reproducible and representative portion of a sample into an atomiser. It depends onthe physical and chemical state (i.e. solid, liquid or gas) of the analyte and the sample

matrix such as soil, water, blood, plant leaves, etc. For solution and gaseous samples,

the introduction step is quite simple but for solid, it poses a major problem.

You have learnt in Unit 7 that a simple method of sample introduction into the flame

is nebulisation. It is a process of thermal vaporisation and dissociation of aerosol

particles at high temperatures thus producing small particle size with high residencetime. Some nebulisation methods are given below.

pneumatic nebulisation ultrasonic nebulisation electrothermal vaporisation hydride generationYou have learnt about pneumatic nebulisation and the nebulisers in Unit 7; let us

learn about the other three types of nebulisers.

Ultrasonic nebulisation: In this case the sample is pumped on to the surface of apiezoelectric crystal that vibrates at a frequency of 20 kHz to a few MHz (Fig. 9.12).

The waves so produced are very efficient in turning the sample into a fine aerosol,

which is carried by a stream of argon, first through a heated tube and then to arefrigerated tube to condense out the solvent. Such nebulisers produce more dense

and homogeneous aerosols because of desolvation and there is no cooling effect. Thisimproves detection limit by a factor of 10 to 20 as compared to pneumatic nebulisers.


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Fig. 9.12: Schematic diagram of an ultrasonic nebuliser

Electrothermal vaporisation: An electrothermal vaporiser (ETV) is an evaporator

located in a closed chamber through which an inert gas such as argon flows to carrythe vaporised sample into the atomiser. The sample can be vaporised from an ETV on

a conductor such as carbon rod or a tube furnace or a heated metal filament

commonly used in AAS. A schematic diagram of a Lvov platform used aselectrothermal vaporiser is given in Fig. 9.13. In contrast to the nebuliserarrangements, ETV system produces a discrete signal rather a continuous one so that

signal from the atomised sample increases to a maximum and then decreases to zero

as the sample is swept through the atomiser. Each preheated electrode is introducedinto the aperture of a preheated cuvette and heated by means of a separate power

supply. It enables 1-200 L samples to yield approximately an order of improvement

in detection limit in the range 0.1-500 pg. The major problems of electrothermalatomisation system are interferences due to sample matrix.

Fig. 9.13: Schematic diagram of Lvov platforman electrothermal vaporiser in a

graphite furnace

Hydride generation technique: This provides a method for introducing samples

containing As, Sb, Sn, Se, Bi and Pb into an atomiser as their representative hydrides.This enhances detection limits by a factor of 10 to 100. As some of these elements are

highly toxic and occur in the environment at very low levels, their determination at

low concentrations is extremely important. The volatile hydrides of these elementsmay be generated by the reaction of an acidified aqueous solution of the sample to

1% aqueous solution of NaBH4 in a glass vessel. A representative reaction of As (III)

with NaBH4 to form arsine (AsH3) is as follows.

3NaBH4+3HCl+4H3AsO3 3H3BO3+4AsH3+3H2O+3NaCl

The volatile hydrides such as AsH3, BiH3, SbH3, H2Se etc. are swept out of thesolution into the atomisation chamber by an inert gas carrier. The chamber is usually

a silica tube heated in a tube furnace or in a flame where hydride gets decomposedleading to the formation of analyte element whose concentration is then determined


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Atomic Absorption

Spectrophotometryfrom the atomic absorption signal. The signal is a peak similar to that obtained with

electrothermal atomisation. Schematic diagram of basic system used for the hydridegeneration and atomisation is shown in Fig. 9.14.

Fig. 9.14: Schematic diagram of a basic system for hydride generation technique

Commercial hydride generation system use either electrically heated or flame heated

quartz tube for atomisation. Its main advantage is enhanced sensitivity and freedomfrom matrix interferences as the element is separated from all other accompanying

elements. In Table 9.2 are given the comparison of detection limits by hydride

generation and graphitic furnace AAS.

Table 9.2: Comparison of detection limits of for selected elements by hydride

generation AAS with graphite furnace AAS

Limit of Detection (g/L)Element

Hydride generation Graphite furnace

As 0.01 0.3

Sb 0.02 0.2

Bi 0.02 0.2

Se 0.01 1.0

Sn 0.04 0.2

SAQ 6

What is the principle of ultrasonic nebuliser?

..

..

..

..

9.8 APPLICATIONS OF ATOMIC ABSORPTIONSPECTROPHOTOMETRY

Atomic absorption spectrophotometry (AAS) is now a routinely and widely employedtechnique for trace and ultratrace analysis of complex matrices of geological,


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biological, environmental, industrial, glass, cement, marine sediment, pharmaceutical,

engine oil or any other kind of samples. It has been employed for the determination ofmore than 60 elements at trace and ultratrace levels. It is frequently used for the cases

where the sample size is small e.g. in case of metalloproteins.

Accuracy in AAS method is generally limited by random errors and noise to about

0.5 5%. Spectral and chemical interferences may however cause systematic errors.

Precision of AAS measurements is typically 0.3 1% at absorbance larger than 0.1 or

0.2 for flame atomisation and 1 5% with electrothermal atomisation. The detection

limits and sensitivities provide a means of comparing characteristics of AAS for agiven element. The detection limits of the AAS method lie in the range of ppb; theGFAAS giving better detection limits as compared to the flame AAS. The detectionlimits of the two methods along with the resonance lines for some commonly

determined elements is compiled in Table 9.3.

The data in Table 9.3 gives only representative detection limits which may vary withthe analyte matrix, nebulisation conditions, flame temperature, sample path length,positioning of burner and other factors including interferences. You would learn in

details about the applications of atomic absorption spectrometry in Unit 11 along withthat of atomic emission spectrometry.

Table 9.3: The resonance lines and approximate detection limits of some selected

elements by flame AAS and GFAAS

Detection Limit (ppb)Element Resonance

line (nm)

Air-acetylene

flame

Graphite furnace

Ag

Ba

328.1

553.6

0.9

8

0.005

0.1

Ca 422.7 2 0.3

Cd 228.8 5 0.1

Cr 357.9 5 0.5

Cu 324.5 4 0.1

Fe

Hg

248.3

253.7

4

200

3

1

K 766.5 4 1

Mg

Mn

Na

285.2

279.5

589.0

3

1

0.2

0.05

0.1

0.05

Ni

Sb

Ti

232.0

217.6

364.3

5

30

.0.2

1

0.2

0.05

Zn 213.9 1 0.006

As AAS is a sensitivetechnique, all possiblesources of contamination

such as due to storage

containers, impurities inreagents, and solvents and

incomplete removal of

earlier sample from the

nebuliser system shouldbe avoided.


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Atomic Absorption

SpectrophotometryMerits and Limitations of Atomic Absorption Spectrophotometry

Some of the merits of atomic absorption spectrometry are as given below.

The equipment is easy to use It is a robust technique The techniques has a small turn around time; of the order of few seconds Moderate cost of analysis per sample Low detection limitsSome limitations of AA spectrophotometry are given below.

Requirement of furnace for the analysis of refractory elements Use of flammable gases Non-automated analytical procedureLet us summarise what have we learnt in this unit.

9.9 SUMMARYAtomic absorption spectrophotometry (AAS) concerns the absorption of radiation by

the atomised analyte element in the ground state. The atomisation is achieved by thethermal energy of the flame or electrothermally in an electrical furnace. Thewavelength(s) of the radiation absorbed and the extent of the absorption form the

basis of the qualitative and quantitative determinations respectively. As atomic

absorption spectrophotometry is not an absolute method of analysis, the routineanalytical methodology for quantitative determinations using AAS is based on

calibration method. Besides this the internal standard method and standard additionmethods are also employed.

A typical atomic absorption spectrophotometer consists of a source delivering the

characteristic resonant radiation of the analyte, an atom reservoir into which the

analyte is introduced and atomised, a monochromator, a detector and a readoutdevice. In a typical flame atomic absorption spectrophotometric determination, the

radiation from a hollow cathode lamp (or electrodeless discharge lamp) is made to fallon the sample of the analyte aspirated into the flame (or in the cuvette of a Lvovgraphite furnace), where a part of it is absorbed. The transmitted radiation is then

dispersed by a monochromator and sent to the detector. The detector output is suitably

processed and is displayed by appropriate readout device. Like, UV-VISspectrophotometers the atomic absorption spectrophotometers are also of two typesviz., single beam atomic absorption spectrophotometers and double beam atomic

absorption spectrophotometers

GFAAS is a much more sensitive as compared to flame AAS and requires a very

small sample size. More so, it does not require any sample preparation; even solid

samples can be analysed without dissolution. However, the background absorptioneffects are quite serious. These are generally sorted out 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 behaviour and thereby tackle the

problem of background. Sometimes the added matrix may modify the analyte also.

Three types of interferences viz., spectral, chemical and physical interferences areencountered in AAS. These need to be suitably addressed to so as to put the techniqueto analytical use. Sample preparation is a crucial step in the AAS determination.

Though in principle, the sample in solid, liquid or in the gas phase can be analysed by

flame AAS but in practice the sample is taken in the solution form. The solution of


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the solids is generally prepared by wet dissolution method using a suitable acid. The

presence of organic solvents of low molar mass e.g. alcohols, ethers, ketones andesters are found to enhance absorption peaks and hence increase sensitivity. A

microwave digestion system (MDS) offers more rapid and efficient decomposition of

complex matrices of geological and biological samples. It greatly reduces the operatortime to prepare samples for analysis. More so, it can be easily automated also.

The accuracy, precision and detection limits of flame AAS depend on how the analyte

sample is introduced into the atomiser. We need to transfer a reproducible and

representative portion of a sample into an atomiser which depends on the physical and

chemical state of the analyte and the sample matrix. The sample introduction isachieved with the help of a nebuliser. The commonly used nebulisation methods arepneumatic nebulisation, ultrasonic nebulisation, electrothermal vaporisation andhydride generation.

Atomic absorption spectrophotometry (AAS) is now a routinely and widely employed

technique for trace and ultratrace analysis of complex matrices of geological,biological, environmental, industrial, glass, cement, marine sediment, pharmaceutical,engine oil or any other kind of samples. The atomic absorption methods using flame

are rapid and precise and are applicable to about 67 elements. Electrothermal methodsof analysis on the other hand are slower and less precise; however, these are more

sensitive and need much smaller samples.

9.10 TERMINAL QUESTIONS1. Why a sharp line source is required in atomic absorption spectrophotometry?2. Why atomic absorption spectrophotometry is not an ideal method for the

determination of alkali metals? Which atomic spectrometric method would yourecommend for these elements?

3. How does the hydride generation method of sample introduction improve thesensitivity of some elements?

4. What do you understand by matrix modifier? What is its importance?5. Why do furnace atomisers provide enhanced sensitivity over flame atomisers in

AAS measurements?

6. Explain the role of organic solvents in atomisation.7. In what way is the signal obtained in GFAAS different from that obtained in

flame AAS?

9.11 ANSWERSSelf Assessment Questions

1. In principle the absorption of radiation in AAS is directly proportional to theconcentration i.e., the Lambert-Beers law holds. However, a number ofexternal factors like the background emission, the flame radiation and other

types of interferences cause deviations from the law. In such a case a

dependable determination can be obtained only with the help of a calibrationplot.

2. The atomic absorption spectrophotometers generally use line sources. Twocommonly used line sources are hollow-cathode lamp (HCL) and electrodelessdischarge lamps (EDL).


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Atomic Absorption

Spectrophotometry3. In graphite furnace the electrothermal heating is done in three stages. In the first

stage the temperature is adequate for evapourating the solvent from the samplesolution. In the next stage the sample is ashed and then finally it is vaporised

producing metal atoms. This three stage heating is called heating cycle.

4. Double beam atomic absorption spectrophotometers are better than a singlebeam spectrophotometer because these correct the fluctuations in the intensity

of radiation coming from the radiation source and for changes in the sensitivity

of the detector.

5. a) Phosphate interferes in the quantitative determination of calcium as itforms calcium phosphate having low volatility which decreases atomicpopulation in the cell.

b) The interference of phosphate in the determination of Ca can be managedby using a releasing agent like Sr or La. These act by reactingpreferentially with the phosphate.

6. The ultrasonic nebuliser uses a piezoelectric crystal which vibrates at extremelyhigh frequencies and the waves so produced turn the sample into a fine aerosol.

The aerosol is then carried by a stream of argon, first through a heated tube and

then to a refrigerated tube to condense out the solvent.

Terminal Questions

1. When the bandwidth of the primary radiation is low with respect to the profileof the analyte absorption, a given amount of analyte would absorb more

radiation. Therefore, the radiation sources having low widths of the emittedanalyte lines are preferred. The continuous radiation sources on the other hand

have low radiant densities and do not provide sufficiently high sensitivity.

2. Alkali metals have low ionisation energies and even at the low temperaturesobtained in the flame a sufficient amount of the sample may be in the excited or

ionised state. This means that the concentration of the analyte atoms in theground state will not be a true representation of the analyte concentration. As

the AAS method depends on the radiation absorption by the analyte in ground

state it is not ideal for such determinations. Flame photometry would be themethod of choice for such determinations.

3. The hydride generation method of sample enhances the detection limits by afactor of 10 to 100 by converting the analyte element into a volatile hydride.Some of the elements for which this method can be used are As, Sb, Sn, Se, Bi

and Pb.

4. The matrix modifier is a commonly acceptable method used to reducebackground effects in GFAAS. In this method, a reagent is added to the sample

that acts by modifying the sample matrix and thereby reduces the problem of

background. In some cases the modifier may modify the analyte also.

5. The sensitivity of electrothermal AAS is much higher as compared to the flameAAS because in the furnace a much higher concentration of atomic vapour can

be maintained as compared with flames. Furthermore, in this method, thedilution of the analyte by the solvent is avoided as the solvent is evaporated

before the atomisation step.

6. The organic solvents of low molar mass like alcohols, ethers, and ketonesenhance the absorption peaks by increasing rate of aspiration, nebulisation

efficiency, formation of finer droplets, and more efficient evapouration or


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combustion of the solvent. However, their presence makes the sample solution

more dilute and the advantage is lost. Therefore, the analyte is extracted into asuitable solvent and the organic phase is directly aspirated into the flame. This

method increases the atomisation efficiency and eliminates a number of

chemical interferences.

7. A transient signal that lasts for a few seconds is produced in GFAAS whereas inflame AAS a steady absorption signal is obtained.

unit 9 atomic absorption

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