Atomic absorption instrument effects

An A AS method is employed for the determination of lead (Pb) in a sample of adulterated paprika by the introduction of lead oxide (of the same colour). An electrothermal atomic absorption instrument that provides a background correction based upon the Zeeman effect is used. 

When analytical atomic absorption was in its early stages of development much information was circulated concerning the freedom of the method from interference effects. This led early users of the technique to assume no interferences occurred in atomic absorption. In fact, atomic absorption has as many types of interferences as flame emission, although in some cases the magnitude of the interference is smaller. Any factor that affects the ground state population of the analyte element can be classed as an interference, since, in atomic absorption, the concentration of the analyte element in the sample is considered to be proportional to the ground state atom population in the flame. Any other factor that affects the ability of the atomic absorption instrument to read this parameter also can be classed as an interference and proper control of these effects is necessary to obtain correct analytical results. The common types of interferences that occur in atomic absorption are the topics of this section. 

Application of the Zeeman effect to atomic absorption instruments is based on the differing response of the two types of absorption lines to polarized radiation. The Tt line absorbs only that radiation that is plane-qrolarized in a direction parallel to the external magnetic field the a lines, in contrast, absorb only radiation polarized at 90° to the field. 

Figure 9-15 shows details of an electrothermal atomic absorption instrument, which uses the Zeeman effect for background correction. Unpolarized radiation from an ordinary hollow-cathode source A is passed through a rotating polarizer B. which separates the beam into two components that are plane-polarized at 90° to one another C. These beams pass into a lube-type graphite furnace similar to the one shown in Fig-... 

Flame emission spectrometry is used extensively for the determination of trace metals in solution and in particular the alkali and alkaline earth metals. The most notable applications are the determinations of Na, K, Ca and Mg in body fluids and other biological samples for clinical diagnosis. Simple filter instruments generally provide adequate resolution for this type of analysis. The same elements, together with B, Fe, Cu and Mn, are important constituents of soils and fertilizers and the technique is therefore also useful for the analysis of agricultural materials. Although many other trace metals can be determined in a variety of matrices, there has been a preference for the use of atomic absorption spectrometry because variations in flame temperature are much less critical and spectral interference is negligible. Detection limits for flame emission techniques are comparable to those for atomic absorption, i.e. from < 0.01 to 10 ppm (Table 8.6). Flame emission spectrometry complements atomic absorption spectrometry because it operates most effectively for elements which are easily ionized, whilst atomic absorption methods demand a minimum of ionization (Table 8.7). 

Zeeman effect splitting of atomic absorption lines. (Redrawn from Concepts Instrumentation and Techniques in Atomic Absorption Spectrophotometry (R. D. Beaty and J. D. Kerber). Perkin-Elmer,... 

Double-beam atomic absorption spectrophotometers are designed to control variations which may occur in the radiation source but they are not as effective as double-beam molecular absorption instruments in reducing variation because there is no blank sample in flame techniques. 

A convenient method is the spectrometric determination of Li in aqueous solution by atomic absorption spectrometry (AAS), using an acetylene flame—the most common technique for this analyte. The instrument has an emission lamp containing Li, and one of the spectral lines of the emission spectrum is chosen, according to the concentration of the sample, as shown in Table 2. The solution is fed by a nebuhzer into the flame and the absorption caused by the Li atoms in the sample is recorded and converted to a concentration aided by a calibration standard. Possible interference can be expected from alkali metal atoms, for example, airborne trace impurities, that ionize in the flame. These effects are canceled by adding 2000 mg of K per hter of sample matrix. The method covers a wide range of concentrations, from trace analysis at about 20 xg L to brines at about 32 g L as summarized in Table 2. Organic samples have to be mineralized and the inorganic residue dissolved in water. The AAS method for determination of Li in biomedical applications has been reviewed . 

The thermal device used to elevate the temperature consists of a burner fed with a gaseous combustible mixture or, alternatively, in atomic absorption, by a small electric oven that contains a graphite rod resistor heated by the Joule effect. In the former, an aqueous solution of the sample is nebulised into the flame where atomisation takes place. In the latter, the sample is deposited on the graphite rod. In both methods, the atomic gas generated is located in the optical path of the instrument. 

Multielement analysis will become more important in industrial hygiene analysis as the number of elements per sample and the numbers of samples increases. Additional requirements that will push development of atomic absorption techniques and may encourage the use of new techniques are lower detction and sample speciation. Sample speciation will probably require the use of a chromatographic technique coupled to the spectroscopic instrumentation as an elemental detector. This type of instrumental marriage will not be seen in routine analysis. The use of Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) (17), Zeeman-effect atomic absorption spectroscopy (ZAA) (18), and X-ray fluorescence (XRF) (19) will increase in industrial hygiene laboratories because they each offer advantages or detection that AAS does not. 

Atomic absorption spectrometry (AAS) is a virtually universal method for the determination of the majority of metallic elements and metalloids in both trace and major concentrations. The form of the original samples is not important provided that it can be brought into either an aqueous or a non-aqueous solution. This situation has been brought about by considerable improvements in instrumentation and also, perhaps partly as a result of this, a better understanding among analysts of the types of interference effect that may modify the expected response of a given element. 

One new instrument is worthy of note. This is the Hitachi Zeeman effect atomic absorption spectrometer, model 170-70. It provides background correction for nonatomic absorption at all wavelengths through use of the Zeeman effect. It is presently offered only in a carbon furnace conflguration. The cost of the instrument is considerably higher than conventional instruments. 

Marr and coworkers have described a procedure for the microdetermination of antimony in organoantimony compounds by atomic absorption spectrophotometry. They compared air-acetylene and air-hydrogen flames and prefer the latter on account of the lower noise. The effects of varying instrumental and chemical parameters were also studied. 

Analytical Procedure. The cold-trap gas phase mercury detection system was designed and used for both laboratory and shipboard measurements of mercury in seawater. The Coleman Instruments mercury analyzer (MAS-50) was incorporated into the analytical system because of its portable and convenient design. However, the effective use of this simple one-element atomic absorption unit requires scrupulous attention to blank determinations for each seawater sample. For example, the undetected presence of either naturally occurring or sampling induced volatile organics which may absorb at the mercury wavelength in the seawater sample can be a serious error. Such artifacts were observed when acidifled seawater samples were stored in low density polyethylene bottles (21), Therefore, the analytical procedure used to determine the mercury concentration in a seawater sample consists of the following steps ... 

No ordinary monochromator is capable of yielding a band of radiation as naiTOW as the width of an atomic absorption line (0.002 to 0.005 nm). As a result, the use of radiation that has been isolated from a continuum source by a monochromator inevitably causes instrumental departures from Beer s law (see the discussion of instrument deviations from Beer s law in Section 24C-3). In addition, since the fraction of radiation absorbed from such a beam is small, the detector receives a signal that is less attenuated (that is, P —> Pq) nd the sensitivity of the measurement is reduced. This effect is illustrated by the lower curve in Figure 24-17 (page 733). 

Instrumentation for diode laser based AAS is now commercially available and the method certainly will expand as diode lasers penetrating further into the UV range become available, especially because of their analytical figures of merit that have been discussed and also because of their price. In diode laser AAS the use of monochromators for spectral isolation of the analyte lines becomes completely superfluous and correction for non-element specific absorption no longer requires techniques such as Zeeman-effect background correction atomic absorption or the use of broad band sources such as deuterium lamps. 

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