Unit 11 Applications of AAS and AES

UNIT 11 APPLICATIONS OF AAS AND AES

Applications of AAS and AES

Structure 11.1

Introduction Objectives

11.2

Salient features of AAS and AES Salient features of AAS Salient features of AES Comparison between AAS and AES

11.3 11.4

Sample Preparation Applications of AAS Biological Samples Environmental Samples Industrial Samples

11.5

Applications of AES Biological Samples Geological Samples Environmental Samples Industrial Samples

11.6 11.7 11.8

Summary Terminal Questions Answers

11.1 INTRODUCTION In the preceding Units 9 and 10, you have studied about basic principles and instrumental aspects of atomic absorption spectrophotometry (AAS) and atomic emission spectrometry (AES) r espectively. You would recall that atomic absorption spectrophotometry concerns the absorption of radiation by the atomised analyte element in the ground state; the atomisation being 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. On the other hand atomic emission spectrometry concerns the emission of radiation by the suitably excited atomic vapours of the analyte; the atomisation as well as the excitation being achieved by any of the numerous available energy sources such as flame arcs, sparks, or plasmas. Here, the emitted radiation and its intensity form the basis for the qualitative and quantitative applications of the technique. You have also learnt about flame emission spectrophotometry (FES), another atomic emission technique, commonly called as flame photometry in Unit 7. You would recall that it is a simple, rapid and inexpensive method for routine analysis of alkali and alkaline earth metals like, sodium, potassium, lithium, calcium and barium in environmental, clinical and biological samples especially in biological fluids and tissues. In this unit, we take up some of the important applications of atomic absorption spectrophotometry and atomic emission spectrometry. We will begin the unit with recalling the salient features of the two techniques and then take up the applications of AAS and AES. In the next block you would learn about some miscellaneous spectroscopic methods.

Objectives After studying this unit, you will be able to: •

outline the salient features of atomic absorption spectrophotometry and atomic emission spectrometry,

57

Atomic Spectroscopic Methods-II

•

compare and contrast atomic absorption spectrophotometry and atomic emission spectrometry,

•

enlist different areas of applicability of atomic absorption spectrophotometry and atomic emission spectrometry,

•

discuss the merits and limitations of atomic absorption spectrophotometry and atomic emission spectrometry,

•

describe some representative applications of atomic absorption spectrophotometry and atomic emission spectrometry,

•

rationalise the complementary nature of atomic absorption spectrophotometry and atomic emission spectrometry.

11.2

SALIENT FEATURES OF AAS and AES

You know that in atomic spectroscopy, the element present in a sample is converted to gaseous atoms or elementary ions in a process called atomisation which may be brought about by any of the available methods. The absorption of the radiation by the vapourised atoms in the ground state, or emission or fluorescence emission of suitably excited state forms the basis of different types of atomic spectroscopies. Collectively, the atomic spectroscopic methods can be used for the qualitative and quantitative determination of about 70 elements in a wide variety of samples of clinical, biological, and environmental srcin. You have learnt in details about AAS and AES in Units 7,9 and 10. Let us recollect the salient features of the AAS and AES methods before taking up their applications.

11.2.1

AAS concerns the absorption of characteristic analyte radiation by the atomised analyte element in the ground state. The wavelength(s) of the radiation absorbed and the extent of the absorption form the basis of the qualitative and quantitative determinations respectively.

•

In flame atomic absorption spectrophotometry, either an air-acetylene or a nitrous oxide-acetylene flame is used to evaporate the solvent and dissociate the sample into its component atoms.

•

It not an absolute method of analysis; the routine quantitative determinations using AAS are based on calibration method. In addition, the internal standard method and standard addition methods are also employed.

•

Compounds of the alkali metals, some of the heavy metals such as lead or cadmium and transition metals like manganese or nickel are all atomised with good efficiency by flame However, a number of refractory elements like V, Zr, Mo and B do not perform well with a flame source.

•

Graphite furnace atomic absorption spectrophotometry (GFAAS) in which the atomisation is achieved electrothermally, is a much more sensitive method as compared to flame AAS. The higher atom density and longer residence time in the graphite tube improve furnace AAS detection limits by a factor of up to 1000 compared to flame AAS. The detection limits may extend to the sub-ppb range.

•

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

•

58

Salient Features of AAS

•

preparation; even solid samples can be analysed without dissolution. The background absorption effects in GFAAS are managed by diluting the sample or selecting another resonance wavelength line. In matrix modifier method a reagent is added to the sample that may modify the matrix behaviour and thereby tackle the problem of background.

•

Accuracy in AAS method is generally of the order of 0.5 – 5%; the precision being 0.3 – 1% at absorbance larger than 0.1 or 0.2 for flame atomisation and 1 – 5% with electrothermal atomisation.

•

It is a robust technique that employs easy to use equipment and can achieve good detection limits. As the turnaround time is small the cost of analysis per sample is not much. However, lack of automation, and usage of flammable gases are not desirable.

11.2.2

Salient Features of AES

•

In atomic emission spectrometry (AES) a reproducible and representative amount of the sample is introduced into an atomization-excitation source wherein it is converted into atomic vapours of the analyte in excited state. The radiation emitted from these is characteristic of the constituents of the sample.

•

The AES is a versatile method due to the availability of a wide range of atomization-excitation sources. Currently, plasma is the most used source. It is high energy source which is an electrically neutral conducting gaseous mixture having a significant concentration of cations and electrons.

•

The plasma can be sustained by supplying energy from a suitable external source. Depending on the power sources employed, there are three different types of plasmas. These are, the inductively coupled plasma (ICP), the direct current plasma (DCP) and the microwave induced plasma (MIP). These plasmas use radiofrequency, direct current and microwave radiation respectively as the power sources.

•

As the energy of the plasma source is quite high, it ensures the excitation of the atoms of all the elements present in the analyte which emit EM radiation characteristic of different elements. Thus, it is a multielement technique.

•

Argon gas is commonly employed as plasma gas due to its inertness, simple emission spectrum, moderately low thermal conductivity, and good natural abundance.

•

Two types of spectrometers are used for ICP-AES. These are sequential spectrometers and simultaneous spectrometers depending on whether the emitted radiation is measured sequentially or simultaneously.

•

In ICP-AES the spectral interference due to the line-rich spectra of the hot plasma source can be minimised by using high resolution spectrometers or using an alternative analyte line. The background effects require the use of offline background correction techniques, or by moving to an unaffected analyte line. The matrix effects are generally handled by using internal standard method.

•

The ICP spectrometers are, however, relatively expensive to purchase and difficult to operate as the user requires extensive training for the maintenance of the instrument.

11.2.3

Applications of AAS and AES

Comparison between AAS and AES

As has already been emphasised, AAS and AES have become the mainstay of the analytical techniques for major, minor and trace element analysis in geological, biological, environmental and industrial samples. Both the techniques can be used for the determination of more than sixty elements, many of which can be determined at 1 ppm level. As regards their applicability, these two techniques are complementary to each other though several points are common amongst them. It must be kept in mind that only metals and metalloids can be determined by usual flame methods like FAAS. This is because resonance lines for nonmetals fall in

59

Atomic Spectroscopic Methods-II

vacuum UV region though some indirect methods have been developed for the same. + For example, chloride can be determined by precipitation with Ag and then either the excess of Ag+ or the one which has already reacted is measured. Similarly phosphorus (525.9 nm) and sulfur (383.7 nm) species exhibit sharp molecular band emission in the Ar-H2 flame. Generally, AAS is considered as more sensitive technique at wavelengths < 300 nm, whereas in visible region, AES is more advantageous. Some elements exhibit maximum sensitivity using molecular band emissions. As the source of radiation in AAS is a hollow cathode lamp which emits the characteristic radiation of a given element, it is a unielemental technique. It is not convenient to measure more than one element at a time by AAS as it is difficult to incorporate more than a single source into the system. Each hollow cathode lamp emits efficiently the spectrum of only one, two, or three elements at a time, measuring additional elements requires substituting a new hollow cathode lamp. Though some advances have been made continuum source yet these arrangements arein somewhat limited asatomic sourcesabsorption extendingspectrophotometry into the ultraviolet region of the spectrum are not widely available. The basic principle of graphite furnace atomic absorption spectrophotometry (GFAAS) is essentially the same as flame atomic absorption spectrophotometry, the only difference being that the atomisation is achieved in a small, electrically heated graphite tube, or cuvette, which is heated to a temperature up to 3000°C to generate the cloud of atoms. The higher atom density and longer residence time in the electrothermal tube improve the detection limits by a factor of up to three orders of magnitude as compared to flame AAS and wecan go down to the sub-ppb range. However, the use of graphite cuvettes, do not sort out the issue of determining refractory elements. It is essential that the AAS instrument should always be calibrated by preparing at least four standard solutions over the concentration range of interest and measuring the absorbance under the same experimental conditions. The correction, if necessary, should be applied to the calibration plot. Sometimes, the method of standard addition is used to compensate for chemical and other interferences. In contrast to atomic absorption spectrophotometry, atomic emission spectrometry is inherently a multielement method. Especially the high temperature of plasma ensures effective atomisation and lead to intense atomic emission. The emission occurs from all elements at the same time and is isotropic. The simultaneous multielement determinations can be made simply by using a multichannel detection system. Multichannel devices using two dimensional spectral dispersion along with two dimensional arrays of detector elements offer extremely good sensitivity and low noise. More so at the operating high temperatures of ICP torch, even the most refractory elements are atomised with high efficiency. As a result, detection limits for these refractory elements can be of the orders of magnitude lower with ICP than with FAAS techniques. These may be at the 1-10 parts per billion level. We can safely generalise the order of detection limits of different techniques as GFAAS (sub-ppb) > ICP-AES (1-10 ppb) > FAAS (sub-ppm). Further, the dynamic range of the various techniques is also important, as it directly affects the amount of dilution required in preparing solutions for analysis. If the dynamic linear range is quite wide, we may use fewer standards. The dynamic ranges 2 3 of FAAS and GFAAS are of the order of only 10-10 only whereas the same for ICP6 AES the dynamic range spreads upto 10. This makes it a suitable technique that is capable of measuring from trace to percent levels. A comparative account of the characteristics of AAS and ICP-AES are briefly summarised in Table 11.1.

60

Table 11.1: A comparative account of the characteristics of AAS and ICP-AES Atomic Absorption Spectrophotometry

Applications of AAS and AES

ICP-Atomic Emission Spectrometry

Primarily a single element technique; though Principally a rapid and multi-element some instruments with multielement sources technique. are available. The flame constituents contribute to the spectral, background and chemical interferences.

Plasma is an optically thin emission source and is relatively free from chemical interferences.

The dynamic range is spread over three orders of magnitude for FAAS and two orders for GFAAS.

The dynamic range is large and extends over a range of 4 to 6 orders of magnitude. It is suitable for analytes from parts per billion to 99.9 per cent.

For AAS the detection limits are in the range Detection limits are generally very low : 1 to of ppm whereas these may go down to sub- 100 ng/g or µg/L (parts per billion). ppb level for GFAAS. Flame AAS is easy to set up and to use, and It falls between these two AAS techniques; requires minimal operator skills, the GFAAS however, it is a bit easier to master than on the other hand is considerably more GFAAS. difficult to operate. FAS procedure cannot be automated whereas it is possible to automate GFAAS.

ICP-AES measurements can be automated.

The accuracy is not very promising.

Good accuracy and precision (relative standard deviation about 1 per cent).

The AAS determinations using flame are It can be used for the determination of most rapid and precise and are applicable to about elements except Ar. In practice, 67 elements. approximately 70 elements can be determined. Not suitable for the elements like, B, C, Ce,

The elements that are difficult to be

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

determined by AAS, can be measured by AES.

11.3

SAMPLE PREPARATION

All samples for determination by AAS or AES must be in solution form except for spark source AES where solids especially metals and alloys with smooth surface can be analysed directly. The detailed procedures for sample preparations have been discussed in Section 9.7 and subsection 10.4.2 respectively for AAS and AES. You would recall that in principle, the sample in solid, liquid or in the gas phase can be analysed by flame AAS but in practice the sample is taken in the solution form. The solution of the solids is generally prepared by wet dissolution method using a 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. On the other hand in ICP-AES, the solution preparation depends on the nature of the sample and the concentration of elements to be determined. The solution for ICP analysis can be prepared either by wet acid method or by direct attack method and suitable precautions are taken as per the requirements of the plasma source.

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Atomic Spectroscopic Methods-II

It must be noted that the possible contamination during dissolution and at the workplace is the most important source of error in the analysis of trace elements and must be avoided. Contamination may come from the air, from the skin of the subject/ sample collector, additives and reagents used in the analysis, as well as parts of instrumentation including glass or plastic wares. Biological materials of human and plant srcin must be handled with extreme care because of sample inhomogeneity especially for trace element analysis. Body fluids such as blood, viscera, urine, etc. additionally need stabilization and homogenization so as to avoid occurrence of any changes in their composition, prior to actual analysis. It is also advisable to keep the total number of transfers to a minimum, and to use accessories made of non-wettable and inert materials.

SAQ 1 What precautions should be observed while preparing samples for AAS and AES? ………………………………………………………………………………………….. ………………………………………………………………………………………….. …………………………………………………………………………………………... …………………………………………………………………………………………...

11.4

APPLICATIONS OF AAS

Atomic absorption spectrophotometry is now a routinely and widely employed technique for trace and ultratrace analysis of complex matrices of geological, biological, environmental or industrial srcin. 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 needmuch smaller samples. Let us take up some applications of AAS in different areas. We begin with biological samples.

11.4.1

Serum is the supernatant liquid of the clotted blood and is separated by centrifugation after addition of anticoagulant such as heparin.

Biological Samples

A wide range of the samples of biological srcin are subjected to analytical procedures for the determination of the elements present in them. These may include plant leaves, fruits, vegetables, blood, urine, muscle tissue, hair, etc. The major difficulty in the analysis of these materials is their complex nature. More so, these samples cannot be analysed directly but require dry ashing followed by wet digestion with oxidising acids such as HNO3 and HClO4. In case of blood analysis, plasma or serum is generally preferred because of the presence of significant amounts of clinically significant elements in them. i)

Determination of calcium in serum

A typical AAS determination of calcium in serum is carried out by calibration method wherein, a calibration plot is obtained by measuring the absorption of characteristic radiation (422.67 nm) for a series of standard solutions of calcium in a similar matrix. An air-acetylene flame is used with a premix burner. The normal calcium content of serum is generally about 100 ppm and it is determined by diluting the sample 1:20 with 1% SrCl2 solution. Thus, a typical sample would contain about 5 ppm of Ca. Therefore, an equivalent amount of sodium and potassium are added to the standard solutions. The plot is then used to determine the concentration of the given sample. It needs to be mentioned that the effects of instrumental parameters and of phosphate and aluminium ion on calcium absorption are to be suitably accounted for an effective determination. Instrumental parameters such as burner height and fuel air ratio may be studied to optimise the experimental conditions of flame height and fuel gas pressure.

62

Similarly the effect of organic solvent such as ethanol may also be studied. The effects of interferants is borne by using 5 ppm each of phosphate, sodium and aluminium solutions

Applications of AAS and AES

In an alternative determination, using the method of standard addition a series of calcium standards of 0, 2.5, 5, 7.5, 10 and 15 ppm are prepared from the 50 ppm stock solution and SrCl2 is added to standards and the unknown to give a final concentration of 1%. The standard addition absorbance calibration plot is then prepared and used for the determination of the concentration of analyte sample. ii)

Determination of cadmium

Cadmium is one of the most important toxic elements from the environmental point of view. It occurs in nature mainly due to volcanic activity. It is used in plating of metals, as stabiliser in polyvinyl chloride, pigments, Ni-Cd batteries and alloying. It is the prime cause of ‘itai-itai’ disease first observed in Japan. Cadmium along with lead has been the most studied element with regard to human toxicology as it has no role in human or plant nutrition. It is highly toxic even in trace amounts to the human body. Total intake of cadmium in Germany, USA and most European countries is in the range, 10-30 µg/day whereas in contaminated areas of Japan, its intake is as high as 400 µg/day. It is most likely to be ingested by tobacco smoking especially cigarettes. Absorption of cadmium is higher in females than in males though its transport in the intestinal tract is influenced by the presence of various food components such as proteins and amino acids. Cadmium in blood may be used as a biological monitoring measure for recent occupational/environmental exposure. In addition, cadmium in urine may also be used as a measure of biological monitoring for body burden where it reflects the total ≤ 1µg/L in the blood of accumulation of cadmium in the body. Typically it occurs at healthy and nonexposed nonsmokers in various countries. Considering the requirements of detection limit and contamination free sample handling, graphite furnace atomic absorption spectrophotometry is the method of choice where the detection limit is 0.04 µg/L. A typical by determination of cadmium in bloodusing involves de-proteination with nitric The acid followed direct determination by GFAAS source with 228.8 nm output. blood sample is collected in plastic collection tubes using vinyl gloves free of talc and o o is stored at a temperature of – 20 C to 4 C. All the laboratory ware is to be soaked in diluted nitric acid and biodistilled quality water is used for dilution work. The determination is preceded by the obtaining calibration curves using matrix adapted calibration solutions. In simple words it means that the calibration solution contains all the known components of the analyte sample. A multiple standard calibration is preferred. Similarly, Cd could also be determined in urine, hair and other body tissues. iii)

Quality control is usually carried out by using certified reference materials from NIST (USA) and NIES (Japan).

Determination of lead

As you know, lead is another highly toxic element which is an environmental contaminant. It enters into biological systems like plants and animals and reaches blood, urine, teeth, bones, hair, plant and animal tissues, etc. These materials need to be analytically assessed for the amount of lead so that its damage potential can be ascertained. From the viewpoint of occupational and environmental toxicology the determination of lead in blood is the most important since the concentration of lead in whole blood is considered to be the best indicator of current lead exposure in humans. It enters into human blood because of inhalation of polluted air, food and water though these are less relevant for assessing health hazards for humans than the amount of lead actually absorbed. In a typical lead determination, after adding heparin, a natural anticoagulant, the blood is treated with trichloroacetic acid to precipitate proteins. These are then separated by

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centrifugation. In order to avoid interferences, lead is extracted in an organic solvent methyl isobutylketone (MIBK) after adding ammonium pyrrolidinedithiocarbamate (APDC) at pH 3. The lead is extracted as Pb (APCO) 2. The organic phase is then aspirated into air-acetylene flame for the determination of lead. The detection limit of lead in blood is 0.1 µg/mL. Most values of lead in blood are in the range 0.3– 0.4 µ g/mL with 0.6 µ g/mL as the upper limit. It is essential that internal and external quality control should be used for the determination of lead in blood. iv)

Zinc in plant leaves

Zinc is an essential nutrient in plants and remains distributed in all parts of the plant. About 1g dried plant leaves are grounded with pestle and mortar and dry ashed in silica crucible at 500ºC. The ash is then dissolved in acid and final solution is prepared to 0.1 M HCl. The solution is then directly aspirated into an air-acetylene flame of AAS. A blank is also prepared in exactly similar manner.

11.4.2

Environmental Samples

Air, soil and water are three components of environment where determination of toxic contaminant is of extreme importance. Analysis of particulate matter in air from industrial establishments is the most representative study of environmental samples by atomic sp ectrometry. i)

Analysis of airborne particulate matter

In the analysis of airborne particulate matter, the choice of sample collection location and collection procedure is very important. For example samples may be collected from surrounding areas of a factory emitting harmful gases affecting workers health adversely or a residential colony located near industrial establishment where toxic pollutants may travel and thus affect general public. A measured volume of air is collected on a cellulose acetate membrane filter using air sampler. Weighed amount of particulate matter is scratched out of the filter paper or the filter paper itself may be dry ashed in a low temperature furnace so as to avoid loss of volatile elements. The particulate matter or ash is then dissolved using acid digestion method and heating on hot plate. The final solution is prepared in dilute HCl and making up final volume to a fixed volume. Appropriate hollow cathode lamps are selected depending on the elements to be determined and respective standard solutions are prepared. Calibration plots should be drawn for each element to be determined and the test solution is aspirated. Thus concentration of desired elements in air dust particulate matter may be determined. Results are usually reported in terms of µg/m3 of air. ii)

Mercury in air/water

Metallic mercury is important as it forms amalgam with other metals. Its alloy with silver was earlier used by dentists for dental filling though it is no longer so because of toxicological effects known since many years.Mercury in air is collected by bubbling air through an acidic KMnO4 solution where volatile elemental mercury is trapped by 2+ oxidising it to Hg . The excess permanganate is reduced with hydroxylamine, and the collected mercury (or mercury in a water sample) is then reduced to the element by SnCl2 according to following equations. 0

–

5Hg + 2MnO

+

4

+ 16H

–

→ +

6MnO 4 + 5NH 2OH + 13H 2+

–

→

→

2+

5Hg

2+

+ 2Mn + 8H2O 2+

6Mn

+ 5NO–3 + 14H2O

0

Hg + SnCl 2 + 2Cl Hg + SnCl 4 As elemental mercury has appreciable vapour pressure at room temperature, and by bubbling argon through the solution, mercury vapour is swept into a quartz ended cell where its atomic absorption is measured at 253.6 nm using mercury line. A calibration curve should be prepared before the sample is run. At least two blanks should also be 64

prepared in the same manner, omitting the addition of mercury. The measured absorbance is corrected for the blanks and the amount of mercury is determined from the calibration curve.

Applications of AAS and AES

Similarly, the water sample from tap, river, or other sources can be analysed. Tap water should contain around 1 ppb or less mercury. In such determinations the water samples and the standards should be run in a similar manner. As in the case of air samples, the correction should be made for the reagent blank as its magnitude will generally govern the detection limit of the procedure. Extreme care must be taken to minimise reagent and glassware contamination. From the calibration graph the quantity and concentration of mercury in the sample is determined. iii)

Trace element contamination in soil

In order to determine toxic contaminants in soil samples, the representative samples are collected from surface of the soil, and also at some depth. These are then passed through a sieve to make it uniform sample and is stored in separate containers to avoid cross contamination. In order to determine all the contaminants, the sample is prepared by treating a weighed amount of soil with 1:1 nitric acid and making appropriate solution. Metal contents such as Ni, Cu, Zn, Cd, Pb, etc. are then determined by using appropriate hollow cathode lamp and air-acetylene flame. It is essential that the standard solutions for each element are prepared in appropriate concentration range and their respective calibration plots are obtained.

11.4.3

Industrial Samples

Quality control of finished products of steel industry and other products such as alloys requires accurate analysis. For such an analysis an alloy or steel is be dissolved in acid (HCl, HNO3, HClO4 or a mixture of these) and a solution is prepared for analysis by AAS. Care must be taken to eliminate excess of acid. A typical example is described in following lines. i)

Determination of molybdenum in steel

Weighed amount of sample is dissolved in aqua regia and finally in dil HCl. Final volume is made up to fixed volume in a volumetric flask by adding doubly distilled water. Molybdenum can be determined in acetylene-air or acetylene-N 2O flame selecting a wavelength of 313.26 nm. Let us take up an example. Example

0.32 g stainless steel sample was dissolved in nitric acid and the resulting solution was 3 made to 100 cm with water. Five standards and the sample solution were aspirated into flame for the determination of nickel. The following observations were made. Concentration of Nickel (ppm)

Absorbance

2

0.126

4

0.250

6

0.374

8

0.500

10 Sample

0.626 0.226

Calculate the percentage of nickel in steel sample.

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

According to the calibration plot the sample concentration is found to be 3.612 ppm. It corresponds to 1.12%. ii)

Tin in canned fruit juice

Availability of fruit juice in tinned cans is becoming increasingly common though it is also marketed in cartons. As tin is likely to be leached in acidic medium of juice the contents of the can get contaminated. Hence its determination is quite important to ascertain the contamination, if any. Sn can be determined successfully by graphite furnace atomic absorption spectrophotometry (GFAAS). In a typical determination, the juice solution is prepared in dil HCl and boiled till it becomes clear. In case the solution is still turbid or colloidal then it is centrifuged and only supernatant is taken for analysis. As the concentration of Sn in juice is likely to be very low, standards are prepared in the concentration range 25-100 ppb together with acid as blank. The measurement is made at a wavelength of 224.6 nm.

SAQ 2 Enlist some important applications of AAS in the area of environmental analysis. ………………………………………………………………………………………….. ………………………………………………………………………………………….. …………………………………………………………………………………………... …………………………………………………………………………………………...

11.5

APPLICATIONS OF AES

Emission spectroscopy is widely used for qualitative as well as quantitative analysis because of high sensitivity and the possible simultaneous excitation of many elements, notably metals and metalloids. AES is especially suited for rapid survey analysis of the elemental contents in small samples at level of 10µ g/g or less. It is essential to construct an analytical curve with known standards. Often the ratio of analyte emission intensity to the emission intensity of a second element contained in, or added to the sample is used. This internal standard method improves the precision of analysis. ICP-AES is used widely for determining trace metals in environmental samples such as drinking water, industrial waste water and ground water supplies and so also for 66

determining trace metals in petroleum products, biological materials, foodstuffs geological samples and industrial quality control. The simultaneous multielemental determinations make it possible to form correlations and to reach conclusions that are impossible with single element determinations. The excellent sensitivity and wide working range for many elements together with the low level of interferences make ICP-AES a nearly ideal method. Let us learn about some typical analytical determinations in different areas using AES.

11.5.1

Applications of AAS and AES

Biological Samples

As you have learnt above, a wide range of the samples of biological srcin are subjected to analytical procedures for the determination of the elements present in them. Let us take up the determination of sodium in serum as a representative example of the application of AES in biological samples. Determination of sodium in serum

Determination of sodium in water or serum is carried out by following the characteristic emission at 589 nm. A calibration plot isprepared between emission intensity and concentration of the standard solutions. The concentration of the sample solution is then determined fromthe calibration plot. In some of the determinations a known amount of an internal standard like lithium is added to the standard solutions as well as the sample solution. The calibration curve is drawn between the emission intensity ratios of the characteristic emissions of sodium to lithium versus the concentration of the standard solutions of sodium.

11.5.2

Geological Samples

The analysis of geological samples constitutes a major area of the application of atomic emission spectrometry. A large proportion of the elements of the periodic table present in the geological samples can be conveniently determined by ICP-AES spectrometry. These are now routinely being measured well within the limits of the methods. In past, a number of analytical methods have been described for the determination of a particular element in a given sample type. These methods could not be used in a generalised way for samples with analytically different matrices. It became difficult to determine the other elements present in the matrix. More so, the elements that are readily detected in mineralised rock samples may not be detectable in non-mineralised samples such as water. However, when ICP-AES is used for analysis of normal silicate rocks, the range of elements that can be measured is large. -1 Only a few of the elements present at concentration above the 10 µg g level are not readily determined by routine ICP analysis. You might know that in the context of rock analysis, there are ten elements that are conventionally quoted as oxide equivalents. These are Si, Al, Fe, Mg, Ca, Na, K, Ti, Mn, P; these can be determined without difficulty. In addition, many of the trace elements such as Li, Sr, Ba, Sc, Y, La, Zr, V, Nb, Cr, Co, Ni, Cu and Zn, that are determined in a routine trace analysis programme can also be conveniently measured by ICP analysis. The detection limits for some typical elements by ICP are compiled in Table 11.2. Further, the trace elements including Mo, Ag, Cd and Hg in mineralised geological samples can readily be determined when they occur at levels above ‘background’ concentration. Lead can also be measured in mineralised samples but not as well at normal levels in silicates (below 20-40 µg/g). The detection limits for Sn, W, U and Th are not good, but concentrations above 50µg/g can be measured. It is also possible to determine the rare earth elements down to subµg/g level in a rock sample using a concentration technique.

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

11.5.3

Detection limit

Element

Detection limit

Ag

0.004

Mo

0.0001

Al

0.00008

Na

0.00002 0.0001

As

0.002

Ni

Au

0.04

P

0.015

B

0.0001

Pb

0.001

Ba

0.00001

Pd

0.0008

Be

0.000003

Pt

0.08

Ca

0.0000001

Rh

0.003

Cd

0.0002

Sc

0.003

Ce

0.0004

Se

0.03

Co

0.003

Si

0.01

Cr

0.0008

Sn

0.003

Cu

0.0006

Sr

0.00003

Fe

0.00009

Th

0.003

Ga

0.0002

Ti

0.00003

Hf

0.01

Tl

0.2

Hg

0.01

U

0.03 0.00006

In

0.03

V

La

0.001

W

0.0007

Mg

0.000003

Zn

0.00001

Mn

0.00002

Zr

0.00006

Environmental Samples

Environment is an important area wherein the elemental analysis is of significant importance. Let us take up the analysis of trace elements in airborne particulate matter by AES as a representative example of this group. Trace elements in airborne particulate matter

AES has been used extensively for the determination of trace elements in atmospheric particulates, especially large scale survey studies where simultaneous multielement analysis is required. Airborne particulate matter is routinely collected by drawing a measured volume of air through filter material such as fiberglass, asbestos, cellulose paper, porous plastic, or graphite in the form of discs or electrodes. However, for the determination of trace elements, the chemical composition of filter is important. For example glass filters show high concentrations of Ba, Sr, Rb, Zn, Ni, Fe, Ca, As and other elements. The composition of filter materials is particularly significant in sampling relatively clean atmospheres because of the low particulate levels collected in reasonable sampling time. The particulates are collected, dried, and weighed; then spectroscopic buffer is added along with internal standards. The sample so prepared is then suitably determined. The detection limits between 0.1 and 5 ng can be obtained for up to 14 elements.

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11.5.4

Industrial Samples

The application of atomic emission spectroscopy in analysing the industrial samples can be illustrated by considering the determination of metals in lubricating oil as discussed below.

Applications of AAS and AES

Determination of metals in lubricating oil

The determination of metals in lubricating oils used in aircraft, truck, locomotive and other engines provide an excellent indication of the mechanical wear and tear of the engine. Infact, as the concentration of certain metals increases, the wearing out parts or components of the engine can be identified resulting in a decision about their replacement or repair. This routine programme ofwear-metal analysis saves lot of money all around the world. The most important wear metals that are monitored are Fe, Al, Mg, Cu and Ag along with other trace metals. Iron appears as an indicator of more than 80 percent of all component failures detected by wear-metal analysis. Aluminium usually relates to wear of oil pumps, cases, housings, pistons and cylinder heads, and copper to wear of bronze parts such as bushings and retainers. Silicon is useful as an indicator of lubricant contamination from dust. These determinations are generally made by spark AES method and the spectra of 10 or more elements in the range of 0.1 to 500 ppm are determined.

SAQ 3 Enlist any five elements present in rock samples that are expressed in terms of their oxide equivalents. ………………………………………………………………………………………….. ………………………………………………………………………………………….. …………………………………………………………………………………………... …………………………………………………………………………………………...

SAQ 4 What do you understand by wear metal analysis? What is its significance? ………………………………………………………………………………………….. ………………………………………………………………………………………….. …………………………………………………………………………………………... …………………………………………………………………………………………... …………………………………………………………………………………………...

11.6

SUMMARY

Atomic absorption spectrophotometry (AAS) and atomic emission spectrometry (AES) are being widely used for the elemental analysis of geological, biological, environmental, industrial and other types of samples. The salient features of AAS and AES along with a comparative account of the two techniques, followed by the sample preparation for the analytical determination by these technique have been recapitulated in this unit to give an overall picture of the two techniques. The importance and principles of some important typical applications of AAS and AES in diverse areas such as biological sample, environmental samples, and industrial samples have also been discussed.

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11.7

TERMINAL QUESTIONS

1.

Explain why simultaneous multielemental determination by ICP-AES is easier compared to that by AAS.

2.

Explain the difference between atomic emission and atomic absorption spectrometry.

3.

4.

5.

6.

Define the following terms; a)

Plasma,

b)

Spectral interference

Explain the observation during AAS determination of uranium where a linear relationship is observed in the concentration range of 500 to 2000 ppm. At lower concentrations, the relationship is nonlinear which, however, becomes linear if 2000 ppm of alkali metal salt is added. Explain the following observations: a)

Atomic emission is more sensitive to flame instability than atomic absorption.

b)

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

c)

Inductively coupled plasmas are suitable for atomic emission spectrometry but it is rarely used for AAS.

Copper was determined in an aqueous sample by AAS following standard addition method. First 10.0 mL each of the sample was pipetted into each of 50.0 mL volumetric flasks. A standard containing 12.5 ppm of Cu were added to the flasks and the solutions were made up to the volume. Following were the absorbances. Standard, mL 0

Absorbance 0.201

10.0

0.292

20.0

0.378

30.0

0.467

40.0

0.554

Plot absorbance as a function of volume of the standard and calculate the concentration of Cu in the sample.

11.8

ANSWERS

Self Assessment Questions 1.

In the process of preparing the sample for AAS or AES, •

70

all possible contamination coming from the air, from the skin of the sample collector, additives and reagents used in the analysis, as well as parts of instrumentation including glass or plastic wares should be avoided.

2.

•

biological materials of human and plant srcin must be handled with extreme care dueto their inhomogeneity especially for trace element analysis.

•

body fluids such as blood, viscera, urine, etc should be stabilised and homogenised so as to avoid occurrence of any changes in their composition, prior to actual analysis.

•

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

Applications of AAS and AES

Some of the important applications of AAS in the area of environmental analysis are as follows •

Analysis of airborne particulate matter

•

Determination of mercury in air/water

•

Determination of trace element contamination in Soil

3.

The following elements are conventionally quoted as their oxide equivalents: Si, Al, Fe, Mg, Ca, Na, K, Ti, Mn, and P

4.

The wear metal analysis refers to the determination of metals in used lubricating oils from the aircraft, truck, locomotive and other engines. This provides an assessment of the mechanical wear and tear of the engine.

Terminal Questions 1.

In case of ICP-AES all the elements get excited at the same time in the plasma torch. The radiation emitted by them can be measured sequentially or simultaneously. Hence it is easier to determine several elements simultaneously. However, in case of AAS a line source hollow cathode lamp is used as the radiation source. As only one analyte element is able to absorb the radiation emitted by the cathode lamp we can measure only one element at a time. For multielemental determination by AAS we need to use a cathode lamp for each element thus making it a difficult task.

2.

Basic difference between atomic emission (AES) and atomic absorption (AAS) spectrometry is the source of radiation and the measured parameter. In AES, the source of radiation is sample itself where the energy for excitation of analyte atoms is supplied by plasma, a flame, an oven or an electric arc or spark. The signal is the measured intensity of the source at the wavelength of interest. On the other hand in case of AAS, the source of radiation is a line source such as hollow cathode lamp. The signal is in terms of absorbance calculated from the radiant power of the source and the resulting power after the radiation has passed through the atomised sample.

3.

a)

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

b)

Spectral interference is due to overlap of lines of an element in the sample matrix with that of an analyte.

ions

4.

Deviations from linearity at low concentrations are often due to significant ionization of the analyte. When an easily ionized element salt such as that of alkali metal is added in excess amount then the ionization of analyte is suppressed because of the electrons produced by ionization of the metal.

5.

a)

In AES, the analyte signal is produced by the small number of excited atoms or ions whereas in AAS the signal is obtained from absorption by much larger number of unexcited species. Any small change in flame

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conditions influence the number of excited species. Whereas such changes have insignificant effect on the number of unexcited species. b)

Monochromator plays an important role in the resolution and selectivity of ICP emission. Thus a high resolution monochromator can isolate the analyte spectral line from other lines and background emission and reduce spectral interferences. In AAS, however, resolution comes primarily from specific line emitted by a hollow cathode lamp. The monochromator isolates only the emission line of the analyte element from lines of impurities and fill the gas where a much lower resolution is needed.

c)

Temperature of inductively coupled plasma is quite high which favours the formation of atoms and ions. Also sample residence times are long so that desolvation and vaporisation are complete. Further atoms and ions are formed in a nearly chemically inert atmosphere. Nearly constant electron concentration to fewer Sinceitexcited state is not formed or leads it is less stable ionization because ofinterferences. high temperature, is not useful for AAS.

6.

72

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

SOME USEFUL BOOKS 1.

Vogel’s Textbook of Quantitative Chemical Analysis by J. Menham, R.C. Denney, J.D. Barnes and M.J.K. Thomas, 6th Edn, Low Price Edition, Pearson Education Ltd, NewDelhi (2000), Ch. 15.

2.

Quantitative Analysis by R. A. Day and A. L. Underwood, 6th Edn, Prentice Hall of India, New Delhi (2001), Ch. 14

3.

Instrumental Analysis, Editors, H. H. Bauer, G. D. Christian and J. E. O’Reilly, 2nd Edn, Allyn and Bacon, Inc., Boston (1991), Ch. 10.

4.

Principles of Instrumental Analysis by D. A. Skoog, F. J. Holler and T. A. Nieman, 5th Edn, Thomson Brooks/Cole, Bangalore (2004).

5.

Fundamentals of Analytical Chemistry by D. A. Skoog, D. M. West, F. J. Holler and S. L. Crouch, 8th Edn, Thomson Brooks/Cole, Bangalore (2004).

6.

Analytical Chemistry by G. D. Christian, 6th Edn, John Wiley & Sons Inc, Singapore (2003), Ch. 15.

7.

Instrumental Methods of Analysis by H. H. Willard, L. L. Merritt, J. A. Dean and F. A. Settle, 7th Edn., CBS Publishers & Disributors, New Delhi (1986) Ch. 10.

8.

Principles and Practice of Analytical Chemistry by F.W. Fifield and D. Kealey, 5th Edn, Blackwell Science Ltd, NewDelhi (2004), Ch. 8.

9.

Modern Methods for Trace Element Determination by C. Vandecasteele and C. B. Block, John Wiley & Sons, New York (1997), Ch. 5.

10.

Handbook of Instrumental Techniques for Analytical Chemistry, Editor, F. Settle, Low Price Edition, Pearson Education Inc, New Delhi (2004), Ch. 20.

11.

Instrumental methods of Chemical analysis by G. W. Ewing, 5th Edn, Mc-Graw Hill, Singapore (1985).

12.

Atomic Absorption Spectrometry, Ed. S. J. Haswell, Elsevier, Amsterdam (1992).

13.

Trace Element Analysis in Biological Specimens, Eds, R. F. M. Herber and M. Stoeppler, Elsevier, Amsterdam (1994)

Applications of AAS and AES

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Atomic Spectroscopic Methods-II

INDEX Absorbance 7, 8, 9, 14, 59, 63, 65 Acid digestion method 40, 64 Advantages and disadvantages of GFAAS 16 Agricultural s cience 51 Analytical methodology in ICP-AES 47 Qualitative analysis using ICP-AES 48 Characteristic line groupings 48 Line coincidences 48 Persistent or RU 48 Spectral line tables 48

Quantitative analysis 48

Appearance of ICP plasma 37 Applications of AAS 62 Biological samples 62 Determination of calcium in serum 62 Determination of cadmium 63 Determination of lead 63 Zinc in plant leaves64

Environmental sa mples 64 Analysis of airborne particulate matter 64 Mercury in air/water 64 Trace element contamination in soil 65

Industrial samples 65 Determination of molybdenum in steel 65 Tin in canned fruit juice 66

Applications of AES 66 Biological samples 67 Determination of sodium inserum 67

Geological samples 67 Environmental sa mples 68 Trace elements in airborne particulate matter 68

Industrial samples 69 Determination of metals in lubricating oil 69

Applications of ICP-AES 51 Agricultural science 51 Environmental science 51 Forensic sciences 51 Geological sciences 51 Health sciences 51 Industry 51 Metallurgy 51

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

Argon gas supply 36 Argon plasma spectroscopy 34 Atomic absorption spectrophotometers 17 Double beam atomic absorption spectrophotometer 18 Single beam atomic absorption spectrophotometer 17

Atomic emission spectrometry based on plasma sources 33 Atomisers 11 Auxiliary gas 35 Background absorption 20 Biological samples 57, 61, 62, 67 Burners 12 Calibration plot method 8 Carbon rod 14 CCD based spectrometers 47 Characteristic line groupings 48

74

Chemical interferences 20 Chemical interferences 50 Choice of argon as plasma gas 38 CID based instruments 47 Comparison between AAS and AES 59 Concentration dependence of absorption 7 Continuum sources 10 Dc electrical source 34 Detectors 13, 39 Direct current plasma 37 Double beam atomic absorption spectrophotometer 18 Dry attack method 41 Echelle spectrometers 46 Electrodeless discharge lamps 11

Applications of AAS and AES

Electrothermal 11, 42 14 Electrothermal atomisers vaporisation Environmental samples 64, 66, 68 Environmental science 51 Filament 14 Flame atomiser 11 Forensic sciences 51 Frit nebuliser 42 Fuel-oxidant ratio 12 Furnace atomic absorption spectrophotometry 14 Geological samples 67 Geological sciences 51 Graphite furnace 15, 58, 60 Graphite furnace atomic absorption spectrophotometry 14 Advantages and disadvantages of GFAAS 16 Carbon rod 14 Electrothermal atomisers 14 Graphite furnace 15

Filament 14 Furnace atomic absorption spectrophotometry 14 Graphite tube 14 Handling background absorption in GFAAS 16 L’Vov furnace 14

Graphite tube 14 Handling background absorption in GFAAS 16 Health sciences 51 Hollow cathode lamp 11 Hydride generation 42 Hydride generation technique 24 Industrial samples 59, 65, 69 Industry 51 Instrumentation for ICP-AES 39 Detector 39 Monochromator 39 Nebuliser 39 Plasma source 39 Processing and readout device 39 Sample Introduction 40 Dry attack method41 Electrothermal Frit nebuliser 42vaporisation 42 Hydride generation 42 Nebulisation 41 Nebuliser 42 Nebulisers for ICP-AES 42

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Atomic Spectroscopic Methods-II

Sample preparation 41 Acid digestion method 41

Ultrasonic 42

Instrumentation for atomic absorption spectrophotometry 10 Atomisers 11 Burners 12 Premix nebuliser-burner 12 Total consumption burner 12 Turbulent flow burner 12

Flame atomiser 11 Fuel-oxidant ratio 12

Detectors 13 Monochromators 13 Radiation sources 10 Continuum sources 10 Electrodeless discharge lamps 11 Hollow cathode lamp 11 Line sources 10

Readout devices 14

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

Interferences in atomic absorption spectrophotometry 19 Chemical interferences 20 Physical interferences 20 Spectral interferences 19 Background absorption 20

Internal standard method 8 L’vov furnace 14 Lambert-beer’s law 7 Line coincidences 48 Line sources 10 Mechanism of plasma formation 36 Merits and limitations of atomic absorption spectrophotometry 27 Metallurgy 51 Microwave digestion 22 Microwave digestion system 22 Microwave frequency generator 34 Microwave induced plasma 38 Monochromators 13, 39 Nebulisation 41 Nebuliser 39, 42 Nebulisers for ICP-AES 41 Persistent or RU 48 Physical interferences 20, 50 Plasma and its characteristics 34 Argon plasma spectroscopy 35 Choice of argon as plasma gas 38 DC electrical source 35 Direct current plasma 37 Inductively coupled plasma 35 Appearance of ICP plasma 37 Argon gas supply 36 Auxiliary gas 35 Mechanism of plasma formation 36 Quartz tube 35 Radio frequency power generators 36 Three electrodes DCP 37 Torch 35 Toroidal plasma 36 Work coil 36

Microwave frequency generator 35

76

Microwave induced plasma 38 Plasma sources 34 Radio frequency generator 35

Applications of AAS and AES

Plasma source 34, 39 Polychromators 45 Premix nebuliser-burner 12 Preparation of the sample 21 Principle of atomic absorption spectrophotometry 6 Concentration dependence of absorption 7 Absorbance 7 Lambert-beer’s law 7

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

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

Processing and readout device 39 Qualitative analysis using ICP-AES 48 Quantitative analysis 48 Quantitative methodology 7 Quartz tube 35 Radiation sources 10 Radio frequency generator 35 Radio frequency power generators 36 Readout devices 14 Rowland circle 47 Sample handling in atomic absorption spectrophotometry 21 Microwave digestion 22 Microwave digestion system 22

Preparation of the sample 21 Sample introduction methods 23 Electrothermal vapourisation 24 Hydride generation technique 24 Ultrasonic nebulisation 24

Scrubbing 21 Use of organic solvents 22

Salient Features of AAS 58 Salient Features of AES 59 Sample introduction 40 Sample introduction methods 23 Sample preparation 40, 61 Scrubbing 21 Sequential spectrometers 44 Simultaneous spectrometers 45 Single beam atomic absorption spectrophotometer 17 Skew scan instruments 44 Solid state array detector spectrometers 46 Spectral interferences 19 Spectral interferences 46, 50 Spectral line tables 48 Standard addition method 9 Three electrodes DCP 37 Torch 35 Toroidal plasma 36 Total consumption burner 12 Turbulent flow burner 12 Types of instruments for ICP-AES 44 Sequential spectrometers 44

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Atomic Spectroscopic Methods-II

Skew scan instruments 44

Simultaneous spectrometers 45 Polychromators 45 Echelle spectrometers 46

Solid state array detector spectrometers 46 CCD based spectrometers 47 CID based instruments 47 Rowland circle 47

Ultrasonic 42 Ultrasonic nebulisation 24 Use of organic solvents 22 Work coil 36

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