Monochromator atomic absorption spectrometry

Describe the factors which cause broadening of spectral lines. In atomic absorption spectrometry, why is it preferable for the source line-width to be narrower than the absorption profile How can this be achieved What are the differing requirements for resolution in monochromators for atomic emission and for atomic absorption spectrometry ... 

This method is based on the emission of light by atoms returning from an electronically excited to the ground state. As in atomic absorption spectrometry, the technique involves introduction of the sample into a hot flame, where at least part of the molecules or atoms are thermally stimulated. The radiation emitted when the excited species returns to the ground state is passed through a monochromator. The emission lines characteristic of the element to be determined can be isolated and their intensities quantitatively correlated with the concentration of the solution. 

In atomic absorption spectrometry (AA) the sample is vaporized and the element of interest atomized at high temperatures. The element concentration is determined based on the attenuation or absorption by the analyte atoms, of a characteristic wavelength emitted from a light source. The light source is typically a hollow cathode lamp containing the element to be measured. Separate lamps are needed for each element. The detector is usually a photomultiplier tube. A monochromator is used to separate the element line and the light source is modulated to reduce the amount of unwanted radiation reaching the detector. 

A. F. Silva, D. L. G. Borges, B. Welz, M. G. R. Vale, M. M. Silva, A. Klassen, U. Heitmann, Method development for the determination of thallium in coal using solid sampling graphite furnace atomic absorption spectrometry with continuum source, high-resolution monochromator and CCD array detector, Spectrochim. Acta, 59B (2004), 841. 

The resolution and selectivity in ICP emission comes primarily from the monochromator. As a result, a high-resolution monochromator can isolate the analyte spectral line from lines of concomitants and background emission. It can thus reduce spectral interferences. In atomic absorption spectrometry, the resolution comes primarily from the very narrow hollow cathode lamp emission. The monochromator must only isolate the emission line of the analyte element from lines of impurities and the fill gas, and from background emission from the atomizer. A much lower resolution is needed for this puipose. 

Fig. 76. Importance of physical line widths in atomic absorption spectrometry, (a) absorption signal for elemental line (b) spectral bandpass of monochromator (c) emission of hollow cathode lamp.

In atomic absorption spectrometry, no ordinary monochromator can give such a narrow band of radiation as the width of the peak of the line of atomic absorption. In these conditions the Beer Law is not followed and the sensitivity of the method is reduced. Walsh demonstrated that a hollow-cathode, made of the same material as the analyte, emits narrower lines than the corresponding lines of atomic absorption of the atoms of the analyte in flame, this being the base of the instruments of atomic absorption. The main disadvantage is the need for a different lamp source for each element to be analysed, but no alternative to this procedure improves the results obtained with individual lamps. 

Figure 3 Principle of construction of atomic absorption spectrometers. (A) Single-beam spectrometer with electrically modulated lamp radiation (B) double-beam spectrometer with reflection and splitting of the primary radiation by a rotating, partially mirrored quartz disk (chopper). 1 - radiation source, 2 -sample cell (atomizer), 3 - monochromator, 4 - detector, 5 -electronics and readout (by permission of Wiley-VCH from Welz B and Sperling M (1999) Atomic Absorption Spectrometry, 3rd, completely revised edition. Weinheim Wiley-VCH).

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

The basic instrumentation used for spectrometric measurements has already been described in Chapter 7 (p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular absorption techniques are summarized in Table 9.1. The principal difference between instrumentation for atomic emission and molecular absorption spectrometry is in the need for a separate source of radiation for the latter. In the infrared, visible and ultraviolet regions, white sources are used, i.e. the energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency transmitter, a tuned detector and no monochromator. 

A flame emission spectrometer therefore consists of an atom source, a monochromator and detector and is therefore simpler instrumentally than the corresponding atomic absorption system. Particular developments engendered by atomic absorption have restimulated interest in flame emission spectrometry after a dormant period. Chief of these is the use of the nitrous oxide—acetylene flame which is sufficiently hot to stimulate thermal atomic-emission from a wide range of metal elements. 

In the case of atomic absorption and atomic fluorescence the selectivity is thus already partly realized by the radiation source delivering the primary radiation, which in most cases is a line source (hollow cathode lamp, laser, etc.). Therefore, the spectral bandpass of the monochromator is not as critical as it is in atomic emission work. This is especially true for laser based methods, where in some cases of atomic fluorescence a filter is sufficient, or for laser induced ionization spectrometry where no spectral isolation is required at all. 

The determination of trace metal impurities in pharmaceuticals requires a more sensitive methodology. Flame atomic absorption and emission spectroscopy have been the major tools used for this purpose. Metal contaminants such as Pb, Sb, Bi, Ag, Ba, Ni, and Sr have been identified and quantitated by these methods (59,66-68). Specific analysis is necessary for the detection of the presence of palladium in semisynthetic penicillins, where it is used as a catalyst (57), and for silicon in streptomycin (69). Furnace atomic absorption may find a significant role in the determination of known impurities, due to higher sensitivity (Table 2). Atomic absorption is used to detect quantities of known toxic substances in the blood, such as lead (70-72). If the exact impurities are not known, qualitative as well as quantitative analysis is required, and a general multielemental method such as ICP spectrometry with a rapid-scanning monochromator may be utilized. Inductively coupled plasma atomic emission spectroscopy may also be used in the analysis of biological fluids in order to detect contamination by environmental metals such as mercury (73), and to test serum and tissues for the presence of aluminum, lead, cadmium, nickel, and other trace metals (74-77). 

Some instruments have been developed for both atomic absorption and atomic fluorescence. However, a powerful source e.g. a laser is required for the latter spectrometry. In principle, when a gaseous metal atom is excited by absorption of radiation, it emits fluorescence radiation when it reverts to the ground state. This can be recorded in a monochromator/detector set up, not unlike atomic absorption. (J.Chem. Educ., 59, 1982,909 895 AnalChem., 53,1981,332A 1448A 54, 1082, 553, 1006A). 

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