Section Atomic Absorption Spectroscopy

The section on Spectroscopy has been expanded to include ultraviolet-visible spectroscopy, fluorescence, Raman spectroscopy, and mass spectroscopy. Retained sections have been thoroughly revised in particular, the tables on electronic emission and atomic absorption spectroscopy, nuclear magnetic resonance, and infrared spectroscopy. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon ICP, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-29, and phosphorus-31. 

Detector Detection in FIA may be accomplished using many of the electrochemical and optical detectors used in ITPLC. These detectors were discussed in Chapter 12 and are not considered further in this section. In addition, FIA detectors also have been designed around the use of ion-selective electrodes and atomic absorption spectroscopy. 

Atomic absorption spectroscopy (AAS) is complementary to atomic emission spectroscopy (see Section 3.5.3) and became available for a wide range of atoms in the mid-1950s. 

Magnesium may conveniently be determined by atomic absorption spectroscopy (Section 21.21) if a smaller amount (ca 4 mg) is used for the separation. Collect the magnesium effluent in a 1 L graduated flask, dilute to the mark with de-ionised water and aspirate the solution into the flame of an atomic absorption spectrometer. Calibrate the instrument using standard magnesium solutions covering the range 2 to 8 ppm. 

A schematic diagram showing the disposition of these essential components for the different techniques is given in Fig. 21.3. The components included within the frame drawn in broken lines represent the apparatus required for flame emission spectroscopy. For atomic absorption spectroscopy and for atomic fluorescence spectroscopy there is the additional requirement of a resonance line source, In atomic absorption spectroscopy this source is placed in line with the detector, but in atomic fluorescence spectroscopy it is placed in a position at right angles to the detector as shown in the diagram. The essential components of the apparatus required for flame spectrophotometric techniques will be considered in detail in the following sections. 

The data presented in Table 21.4, in conjunction with the experimental details given in Sections 21.21-21.26, will enable the determination of most elements to be carried out successfully. For detailed accounts of the determination of individual elements by atomic absorption spectroscopy, the Bibliography (Section 21.27) should be consulted. In addition, most instrument manufacturers supply applications handbooks relative to the apparatus in which full experimental details are given. 

Depending on the circumstances at hand, several different types of mean comparisons can be made. In this section we review the method for comparison of two means with independent samples. Other applications, such as a comparison of means with matched samples, can be found in statistical texts. Suppose, for example, we have two methods for the determination of lead (Pb) in orchard leaves. The first method is based on the electrochemical method of potentiometric stripping analysis [1], and the second is based on the method of atomic absorption spectroscopy [2], We perform replicate analyses of homogeneous aliquots prepared by dissolving the orchard leaves into one homogeneous solution and obtain the data listed in Table 3.1. 

Flame atomic absorption spectroscopy (AAS) is currently the most widely used of all the atomic methods listed in Table 28-1 because of its simplicity, effectiveness, and relatively low cost. The technique was introduced in 1955 by Walsh in Australia and by Alkemade and Milatz in Holland. The first commercial atomic absorption (AA) spectrometer was introduced in 1959, and use of the technique grew explosively after that. Atomic absorption methods were not widely used until that time because of problems created by the very narrow widths of atomic absorption lines, as discussed in Section 28A-1. 

Speciation of organometallic compounds containing Au and Ag, on the other hand, was paid only scant attention as a subject for analytical research. The analytical methods that supported research on these compounds had to be dug out from the experimental sections of articles dealing with diverse aspects of chemistry, physics, biochemistry, etc. A review appeared on some of the antiinflammatory drugs shown in Table 1, discussing determination of Au in body fluids and pharmaceutical preparations, and speciation of the compounds and their metabolites. The methods included varieties of atomic absorption spectroscopy (AAS), varieties of neutron activation analysis (NAA), inductively coupled plasma spectrometry combined with mass spectrometric detection (ICP-MS),... 

The next component part shown in Figure 2.54 is the monochromator. The monochromators used in atomic absorption spectroscopy are the same as for all the other spectral analytical methods. A detailed description of the monochromators can be found in the section in this chapter devoted to infrared spectroscopy. 

The measurement of total calcium in a biological sample can be made by any method sensitive only to the element and not to its particular chemical form. Atomic absorption spectroscopy is excellent as such a method. Obviously, the spatial resolution that can be obtained with this method is limited, and it is hard to imagine its application to elemental mapping of single cells. The techniques discussed in this subsection have been limited to those that permit a spatial resolution of at least 1 u.m on samples usually prepared by sectioning the frozen biological specimens. 

Excitation of the outer ns electron of the M atom occurs easily and emission spectra are readily observed. We have aheady described the use of the sodium D-line in the emission spectrum of atomic Na for specific rotation measurements (see Section 3.8). When the salt of an alkali metal is treated with concentrated HCl (giving a volatile metal chloride) and is heated strongly in the non-luminous Bunsen flame, a characteristic flame colour is observed (Li, crimson Na, yellow K, lilac Rb, red-violet Cs, blue) and this flame test is used in qualitative analysis to identify the M ion. In quantitative analysis, use is made of the characteristic atomic spectrum in flame photometry or atomic absorption spectroscopy. 

Sources that emit a few discrete lines find wide use in atomic absorption spectroscopy, atomic and molecular fluorescence spectroscopy, artd Raman spectroscopy (refractomeiry and polarimciry also u.se line sources). The familiar mercury and sodium vapor lamps provide a relatively few sharp lines in the ultraviolet and visible regions and are used in several spectroscopic instruments. I loilow-cathodc lamps and clectrodelcss discharge lamps are the most important line sourcc.s for atomic absorption and fluorescence methods. Discussion of such sources is deferred to Section 9H- . 

The standard-addition method, which was described in Section ID-3, is widely used in atomic absorption spectroscopy lo partially or completely compensate for the chemical and spectral inlcrfcrcncc.s introduced by the. sample matrix. 

Why did dc polarography rapidly disappear from analytical laboratories in the mid-1950 s The main application of dc polarography in analysis was to heavy metal cation analysis and the detection limit here is about 10 mol dm - (ca 6 ppm for copper). In the mid 1950 s atomic absorption spectroscopy (AAS) became available and was routinely used to analyse for heavy metal cations at <1 ppm. Atomic absorption spectroscopy, although more expensive in initial costs, is an easier technique to use than polarography. Atomic absorption spectroscopy became rapidly established as the method of choice and has remained so until the present day. It is only in recent years that advanced forms of polarography using pulse techniques have begun to become competitive with AAS (see Section 3.0 and 4.0). 

For analytical purposes it is essential that interference effects in atomic absorption spectroscopy be eliminated or minimized. If they cannot be eliminated, it is necessary to compensate adequately for their presence through use of proper standards or other compensating techniques. This section deals with this problem. Reference also should be made to Chapter 9, as many techniques used for flame emission also apply to atomic absorption. 

Reference should be made to Section 7 in Chapter 9, since many of the techniques described there also are applicable to atomic absorption spectroscopy. As in flame emission, a variety of read-out devices are used for atomic absorption. These include visual meters, chart recorders, digital read-out, devices that convert data to absorbance units, and even digital devices that can be calibrated to present analytical data directly in concentration units. The method used to process the read-out data depends on the nature of the read-out signal. In the case of direct read-out in concentration units the data require no further processing. Such data may be fed to the input of a computer for a direct printing of analytical results. 

Most of the processes in the flame that interfere with fluorescence intensity are similar to those that occur in flame emission and atomic absorption spectroscopy. The reader is referred to sections dealing with this topic in Chapters 9 and 10. 

The same general approach used in solving these dilution problems can be used when concentrations are expressed in units other than molar concentration. Concentrations of metals dissolved in water are frequently measured by atomic absorption spectroscopy, a technique that depends upon absorption of light by atoms. To measure the concentration of metal in an unknown solution, it is necessary to have a standard solution (see Section 7.6) of known concentration. As purchased, these standard solutions contain 1000 mg of the desired metal dissolved in IL of solution. Suppose that one had a standard solution of CaClj containing 1000mg of Ca + per liter of solution. How would this solution be diluted to make 5L of a solution containing 20 mg of calcium per liter ... 

Initially analyze all of the air samples for their cadmium content by flame atomic absorption spectroscopy (AAS) according to the instructions given in Section 3.10. If the concentration of cadmium in a sample solution is less than three times the quantitative detection limit [0.04 jg/mL (40 ng/mL) for the instrumentation used in the validation] and the sample results are to be averaged with other samples for TWA calculations, proceed with the AAS-HGA analysis of the sample as described below. 

The air-dried spiked filters were digested and analyzed for their cadmium content by flame atomic absorption spectroscopy (AAS) following the procedure described in Section 3. The 0.02 to 2.0 pg/mL cadmium standards (the suggested working range) were used in the analysis of the spiked filters. 

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