Unit 9 Atomic Absorption

5 Atomic Absorption Spectrophotometry

UNIT 9 ATOMIC ABSORPTION

SPECTROPHOTOMETRY

9.1 Introduction

Objectives

9.2 Principle of Atomic Absorption Spectrophotometry

Concentration Dependence of Absorption Quantitative Methodology

9.3 Instrumentation for Atomic Absorption Spectrophotometry

Radiation Sources Atomisers Monochromators Detectors Readout Devices

9.4 Graphite Furnace Atomic Absorption Spectrophotometry

Electrothermal Atomisers

Handling Background Absorption in GFAAS Advantages and Disadvantages of GFAAS

9.5 Atomic Absorption Spectrophotometers

Single Beam Atomic Absorption Spectrophotometer Double Beam Atomic Absorption Spectrophotometer

9.6 Interferences in Atomic Absorption Spectrophotometry

Spectral Interferences Chemical Interferences Physical Interferences

9.7 Sample Handling in Atomic Absorption Spectrophotometry

Preparation of the Sample Use of Organic Solvents Microwave Digestion Sample Introduction Methods

9.8 Applications of Atomic Absorption Spectrophotometry 9.9 Summary

9.10 Terminal Questions 9.11 Answers

9.1

INTRODUCTION

You 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 you have learnt about flame photometry and atomic fluorescence spectrometry. In flame photometry 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 absorption spectrophotometry (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) of the radiation absorbed and the extent of the absorption form the basis of the

qualitative 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

6

Atomic Spectroscopic

Methods-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 instrumentation required 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 absorption spectrophotometry 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 in diverse 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 atomic

absorption spectrophotometer,

• justify the usage of line radiation sources in atomic absorption

spectrophotometry,

• describe the functioning of different types of nebulisers used in atomic absorption spectrophotometry,

• compare the flame and flameless atomisation of the analyte in terms of

sensitivity and detection limits,

• outline the importance of sample handling in atomic absorption

spectrophotometry,

• discuss the interferences observed in atomic absorption spectrophotometric

determinations, and

• state the merits and limitations of the atomic absorption spectrophotometric technique.

9.2

PRINCIPLE OF ATOMIC ABSORPTION

SPECTROPHOTOMETRY

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 the atomisation is performed by aspirating the sample solution into a flame where the analyte element is converted into gaseous phase atoms. Alternatively, the sample is fed into a graphite furnace where the atomisation is achieved electrothermally at relatively 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 vapours

containing 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 from Unit 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 atoms are involved in this process. Therefore, the ionisation occurring due to high temperature of the flame needs to be kept to a minimum.

7 Atomic Absorption Spectrophotometry The 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 being replaced 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 by suitable standard samples of the analyte under similar experimental conditions, i.e., a calibration curve method is generally employed.

9.2.1 Concentration Dependence of Absorption

You 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 most elements 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, page 8, Block 3). The absorption follows Lambert-Beer’s 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-Beer’s law, the extent of radiation absorbed by the absorbing species is a function of the path length and the concentration of the absorbing species.

Mathematically, bc P P

ε

P_{o = radiant power of incident light,} P _{= radiant power of transmitted light,} b _{= thickness of the absorbing medium,}

ε _{= 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.

bc

A

P

P

ε

=

=

log

Thus, absorbance of the sample is directly proportional to the concentration of the analyte. 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 radiation passing 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 nonabsorbed radiation 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 absorption spectrophotometry.

9.2.2 Quantitative Methodology

Like 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 7 in the context of flame photometry. These are briefly recalled here.

Typical absorbance must be in the range 0.1 to 0.3 or else precision is poorer at the extremes due to instrumental noise. In AAS the absorption of resonant radiation by ground state atoms of the analyte is used as the analytical signal.

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Atomic Spectroscopic

Methods-II Calibration plot method

In this method, a calibration plot is drawn by aspirating standard solutions of known concentration 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 Beer’s law is followed in AAS, in practice the departures from linearity 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 the Lambert-Beer’s 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 take 3-4 standards of different concentration and a bank to obtain a suitable calibration plot. 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 or

concentrated.

As the test solution is often a complex whose all constituents are not known, it becomes almost impossible to prepare standard solutions having a similar

composition 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 to the 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 internal standard 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.

9 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 solution of 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 the added concentrations of the analyte. It is then extrapolated to the concentration axis to obtain 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 as a 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 Spectroscopic

Methods-II

9.3

INSTRUMENTATION FOR ATOMIC ABSORPTION

SPECTROPHOTOMETRY

You have learnt above that in AAS the absorption of resonant radiation by ground state atoms of the analyte is used as the analytical signal. Accordingly, a source delivering 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 some other atomiser. A typical atomic absorption spectrophotometer consists of the following components. • Radiation source • Atom reservoir • Monochromator • Detector • Readout device

A block diagram showing the basic components of an atomic absorption spectrophotometer is given in Fig. 9.4.

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

In a typical flame atomic absorption spectrophotometric determination, the radiation from 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 suitably processed 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 simultaneous measurements at two different wavelengths. These contain two independent monochromators 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 AAS

instruments in Section 9.5.

Let us learn about various components of an atomic absorption spectrophotometer. 9.3.1 Radiation Sources

All commercially available atomic absorption spectrophotometers use a radiation source that emits the characteristic spectrum of the element to be determined. The essential 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

11 Atomic Absorption Spectrophotometry was 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 sources are also frequently

employed for the purpose and in fact are superior for the elements such as As, Se and Te 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 are preferred. Accordingly, the radiation sources are designed so as to operate at much lower 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 as

source resolution. Fig. 9.5 illustrates source resolution achieved by using radiation

sources emitting sharper lines.

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

9.3.2 Atomisers

The purpose of atomiser is to provide a representative portion of the analyte in the optical path and convert it into free neutral ground state atoms. In atomic absorption spectrophotometry, the flames and furnaces that generate a temperature in the range of 1500 to 3000 ºC 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 will learn about electrothermal alone use in flameless atomic absorption spectrum.

Flame atomiser

You have learnt about flame atomisers in Unit 7. In a typical flame atomisation process, 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 of a burning gas and an oxidant is directed into a suitable burner. As already described in Unit 7, Section 7.5, flame temperature depends on fuel-oxidant ratio and the requisite temperature for analysis can be obtained by varying the 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 up newer areas of application of the technique.

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Atomic Spectroscopic

Methods-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 path length 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 for each element in AAS. The fuel-oxidant ratio and observation heights are so chosen as to 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 is sprayed 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 in AAS.

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 preferred for 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,

13 Atomic Absorption Spectrophotometry equipments 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 further diluted by the fuel and oxidant gases so that test material has very small concentration in the flame.

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

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 have been developed for atomisation about which you would learn in the next section. 9.3.3 Monochromators

You know that the monochromators are the devices that can selectively provide radiation 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 all non 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 sample components during atomisation. Most commercial AAS instruments use diffraction gratings as monochromators.

9.3.4 Detectors

As the wavelengths of resonance lines fall in UV region, the most commonly used detector 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 ideal atomisers as for a number of elements the

atomisation is not quantitative. The sensitivity is lowered considerably due to this and by dilution of the analyte atom population with gases in the flame.

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Atomic Spectroscopic

Methods-II not only from the selected resonance lines but also from the emission within the flame. Therefore, in addition to absorption signal intensity IA, the detector may receive 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 modulating the emission from the emission line source using a mechanical chopper device. 9.3.5 Readout Devices

The readout systems include meters, chart recorders and digital display meters. These days, 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 and calibration curves, etc.

SAQ 2

Name the line sources employed in atomic absorption spectrophotometry.

……….. ……….. ……….. ……….. ……….. ………..

9.4

GRAPHITE FURNACE ATOMIC ABSORPTION

SPECTROPHOTOMETRY

As mentioned earlier, the flame atomisation method suffers from some drawbacks. In order 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 L’Vov furnace and the carbon rod or filament. AAS using graphite furnace

is called Graphite Furnace Atomic Absorption Spectrophotometry (GFAAS) which is highly sensitive (100 to 1000 times as compared to flame AAS) and requires a very small sample size. It has a further advantage of not requiring any sample preparation. 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 is converted into vaporised atoms in ground state that are subjected to the characteristic resonance 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 transient signal 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 Atomisers

The use of furnaces as atomisers for quantitative AAS goes back to the work of L’Vov, which led to the breakthrough of atomic absorption spectrophotometry towards very low absolute detection limits. The essence of L’Vov method is to

15 Atomic Absorption Spectrophotometry completely 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 be suitably 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 K whereas 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 about 5 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 µL) 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 µg) is introduced directly into the graphite tube.

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

The nature and design of cuvettes or sample holder is of great importance in GFAAS. Different types of cuvettes are commercially available. The standard cuvette made from electrographite is suitable for the determination of volatile elements such as Pb and 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 temperature capable 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) is followed. 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 tube containing 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 only when the analyte is atomised. You may note the correspondence between the atomization 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 analyses depending upon the nature of material.

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Atomic Spectroscopic Methods-II

(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 is so 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 the analyte by the solvent is avoided as the solvent is evapourated before the atomisation step.

9.4.2 Handling Background Absorption in GFAAS

High 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 this 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. 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 the analyte atomisation.

9.4.3 Advantages and Disadvantages of GFAAS

The most significant advantages of this flameless vaporisation method are: • It eliminates the possibility of the interaction of the sample with different

components of the flame, thereby eliminating anomalous results.

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

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

• It provides an ability to deal with very small sample sizes. This becomes quite important 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 to the contributions from the remains of the previous determinations. While using GFAAS, great care is required during sample preparation because of contamination arising out from

glassware and volumetric pipettes.

17 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 SPECTROPHOTOMETER

Having learnt about the essential components of an atomic absorption

spectrophotometer let us now learn about the different types of atomic absorption spectrophotometers. You would recall the spectrophotometers used in UV-VIS spectrophotometry. It is pertinent to mention here that the atomic and molecular absorption spectrometries are quite similar in principle and instrumentation; the radiation 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 spectrophotometer

9.5.1 Single Beam Atomic Absorption Spectrophotometer

A 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 signal arising 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 atomic line of interest is isolated.

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

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Atomic Spectroscopic

Methods-II The electronic amplifier is synchronised with the chopper so that the signal

component generated by emission from the sample is not detected. The attenuation of the 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 Spectrophotometer

In this instrument the beam from the HCL is split by a mirrored chopper, one half passing 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 passed through 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 for changes 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 the loss 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. Most instruments 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 a simultaneous 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 lamps

emitting 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 work with both flame and furnace atomisers. Improved analytical precision is obtained when an autosampler is used in conjunction with a furnace atomiser. • The monochromator is capable of high resolution typically 0.04 nm, a feature

more 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 of 180 – 800 nm.

19 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 software must optimise flame and spectrometer parameters including furnace

temperature. SAQ 4

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

……….. ……….. ……….. ………..

9.6

INTERFERENCES IN ATOMIC ABSORPTION

SPECTROPHOTOMETRY

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 a chemical, physical or spectral effect that may cause the analyte signal to be altered in intensity. Accordingly, there are three types of interferences in AAS. These are spectral, chemical and physical interferences. Let us learn about these in the contexts of flame AAS and GFAAS.

9.6.1 Spectral Interferences

These 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. It qualifies 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 of other 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 a positive 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 salt content are aspirated. This arises because of incomplete desolvation and is called

background 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 the element itself. The measurements should be made at two other lines from the hollow cathode 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 background correction line. This technique requires two separate measurements on the sample. A yet another source of spectral interference is the background emission from the flame. This may be corrected by modulation of the output of the radiation source and the 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 discussed here, you can obtain information about these from the reference texts listed at the end of the block.

Absorption due to molecular species and scattering are more problematic with electrothermal atomization.

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Atomic Spectroscopic

Methods-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 compounds of low volatility from the analyte element. For example, the refractory elements such as 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 formation of 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 be eliminated 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 La can be used as releasing agents to minimise phosphate interference as these would react preferentially with the phosphate.

9.6.3 Physical Interferences

These 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 be due to variations in the gas flow rates, changes in the solution viscosity affecting its rate 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 the standard. Errors due to changes in viscosity may be avoided by matching the matrix and by performing frequent calibrations. Some instruments offer the capability of using 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 by atomic absorption spectrophotometry?

……….. ……….. ……….. ……….. b) How is this interference handled?

……….. ……….. ……….. ………..

9.7

SAMPLE HANDLING IN ATOMIC ABSORPTION

SPECTROPHOTOMETRY

In principle, the sample in solid, liquid or in the gas phase can be analysed by flame AAS. 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

21 Atomic Absorption Spectrophotometry could be analysed directly also by using an electrothermal furnace. The gaseous

samples, on the other hand, generally are pretreated by scrubbing before the resultant solution. 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 Sample

The 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 by acid 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 the decomposition of silicate materials, however, HF must be used. A combination of nitric acid and perchloric acid is especially useful for the complete destruction of fats and proteins in biological samples.

In a typical dissolution step, a suspension of the sample in acid is heated by flame or a hot plate until complete dissolution i.e., when the entire solid has disappeared and a transparent solution is obtained. The decomposition temperature is the boiling point of acid. However, such a wet decomposition in open vessels may give rise to systematic errors due to volatilisation losses and contamination caused by the reagents 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 closed systems 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, a

concentration 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 out in 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 1 M or else aspiration of corrosive solution may damage the burner.

9.7.2 Use of Organic Solvents

In the early development stages of AAS, it was observed that analyte solutions containing 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 efficient evaporation 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 AAS determination.

22

Atomic Spectroscopic

Methods-II 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 into the flame. This method has following advantages.

• The analyte element is separated from the bulk matrix of the sample thereby eliminating chemical interferences.

• Complexation of metal with organic solvent increases atomisation efficiency causing upto ten fold signal enhancement.

• The analyte element may be extracted into a smaller volume of organic solvent with 10to 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 solvent which must be burned along with the fuel. If the flame is too rich in fuel, the solvent will 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 be followed while using organic solvents.

9.7.3 Microwave Digestion

In 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. The concept of microwave ovens for the decomposition of inorganic and organic samples was first proposed during mid 1970s. This method has an advantage over

conventional methods as it takes less time because of rapid heating ability of microwaves.

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 reached very quickly due to increased pressure in the vessel. In addition, small amounts of reagents are used and evaporative losses are avoided, thus reducing interferences by reagent 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 be simultaneously dissolved.

Microwave digestion vessels are constructed from low-loss materials that are transparent 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 of H2SO4 and H3PO4. For these acids, quartz and borosilicate glass vessels are used. A typical closed vessel microwave digestion system consisting of teflon body, cap and a safety relief valve is shown in Fig. 9.11. The maximum recommended temperature obtained with this device is 250 ºC. When overpressurisation occurs, safety valve gets distorted similar to home pressure cookers and the excess pressure is released.

Microwave region of electromagnetic spectrum corresponds to lower energy (or longer wavelength) than IR radiation. Most commonly used

frequency is 2450 MHz as set by international convention for use for industrial, scientific and medical purposes.

23 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 Methods

The sample introduction into the flame is an important step in flame AAS measurements 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 on the 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 residence time. Some nebulisation methods are given below.

• pneumatic nebulisation • ultrasonic nebulisation • electrothermal vaporisation • hydride generation

You 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 a

piezoelectric 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 a refrigerated tube to condense out the solvent. Such nebulisers produce more dense and homogeneous aerosols because of desolvation and there is no cooling effect. This improves detection limit by a factor of 10 to 20 as compared to pneumatic nebulisers.

24

Atomic Spectroscopic Methods-II

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 carry the 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 L’vov platform used as electrothermal vaporiser is given in Fig. 9.13. In contrast to the nebuliser

arrangements, 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 introduced into the aperture of a preheated cuvette and heated by means of a separate power supply. It enables 1-200 µL samples to yield approximately an order of improvement in detection limit in the range 0.1-500 pg. The major problems of electrothermal atomisation system are interferences due to sample matrix.

Fig. 9.13: Schematic diagram of L’vov platform−−−−an 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 elements may 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 the solution 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 decomposed leading to the formation of analyte element whose concentration is then determined

25 Atomic Absorption Spectrophotometry from the atomic absorption signal. The signal is a peak similar to that obtained with

electrothermal atomisation. Schematic diagram of basic system used for the hydride generation 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 freedom from 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 (µg/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 ABSORPTION

SPECTROPHOTOMETRY

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

26

Atomic Spectroscopic

Methods-II biological, environmental, industrial, glass, cement, marine sediment, pharmaceutical, engine oil or any other kind of samples. It has been employed for the determination of more 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 a given element. The detection limits of the AAS method lie in the range of ppb; the GFAAS giving better detection limits as compared to the flame AAS. The detection limits 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 with the 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 with that 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 sensitive technique, all possible sources of contamination such as due to storage containers, impurities in reagents, and solvents and incomplete removal of earlier sample from the nebuliser system should be avoided.

27 Atomic Absorption Spectrophotometry Merits 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 limits

Some limitations of AA spectrophotometry are given below. • Requirement of furnace for the analysis of refractory elements • Use of flammable gases

• Non-automated analytical procedure

Let us summarise what have we learnt in this unit.

9.9

SUMMARY

Atomic absorption spectrophotometry (AAS) 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) 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 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.

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 readout device. In a typical flame atomic absorption spectrophotometric determination, the radiation from a hollow cathode lamp (or electrodeless discharge lamp) is made to fall on the sample of the analyte aspirated into the flame (or in the cuvette of a L’vov graphite 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-VIS

spectrophotometers the atomic absorption spectrophotometers are also of two types viz., 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 absorption effects 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 are encountered in AAS. These need to be suitably addressed to so as to put the technique to 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

28

Atomic Spectroscopic

Methods-II 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 and esters 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 operator time 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 is achieved with the help of a nebuliser. The commonly used nebulisation methods are pneumatic nebulisation, ultrasonic nebulisation, electrothermal vaporisation and hydride 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 methods of analysis on the other hand are slower and less precise; however, these are more sensitive and need much smaller samples.

9.10

TERMINAL QUESTIONS

1. 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 you recommend for these elements?

3. How does the hydride generation method of sample introduction improve the sensitivity 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

ANSWERS

Self Assessment Questions

1. In principle the absorption of radiation in AAS is directly proportional to the concentration i.e., the Lambert-Beer’s law holds. However, a number of external 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 calibration plot.

2. The atomic absorption spectrophotometers generally use line sources. Two commonly used line sources are hollow-cathode lamp (HCL) and electrodeless discharge lamps (EDL).

29 Atomic Absorption Spectrophotometry 3. 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 sample solution. 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 single

beam 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 it forms calcium phosphate having low volatility which decreases atomic population in the cell.

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

preferentially with the phosphate.

6. The ultrasonic nebuliser uses a piezoelectric crystal which vibrates at extremely high 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 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 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 temperatures

obtained 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 the ground 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 the method of choice for such determinations.

3. The hydride generation method of sample enhances the detection limits by a factor 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 reduce

background 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 flame AAS 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 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 ketones enhance the absorption peaks by increasing rate of aspiration, nebulisation efficiency, formation of finer droplets, and more efficient evapouration or

30

Atomic Spectroscopic

Methods-II combustion of the solvent. However, their presence makes the sample solution more dilute and the advantage is lost. Therefore, the analyte is extracted into a suitable 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 in flame AAS a steady absorption signal is obtained.