Atomic Absorption with a Continuum Source

Figure 14.5—Comparison of transmit ted intensities in atomic absorption with a continuum source (a and b) and with a tamp that emits spectraI lines (c and d). The square region shows the wavelength interval percieved by the photomultiplier tube (PMT). The PMT signal is proportional to the area of the white parts in the squares. In this way, the resolution is in the source , as expressed by Walsh, who is considered to be one of the pioneers of atomic absorption.

J. M. Harnly, The future of atomic absorption spectrometry a continuum source with a charge coupled array detector, J. Anal. Atom. Spectrom., 14 (1999), 137. 

Figure 6.25 (Top row) Atomic and background absorption with a bne source (HCL, EDL). (Bottom row) Atomic and background absorption with a continuum source (deuterium lamp, tungsten-filament lamp). The width (x-axis) of each diagram is the spectral slit width. [FiDm Beaty and Kerber, used with permission of PerkinElmer, Inc. (www.perkinelmer.com).]...

Equation 10.1 has an important consequence for atomic absorption. Because of the narrow line width for atomic absorption, a continuum source of radiation cannot be used. Even with a high-quality monochromator, the effective bandwidth for a continuum source is 100-1000 times greater than that for an atomic absorption line. As a result, little of the radiation from a continuum source is absorbed (Pq Pr), and the measured absorbance is effectively zero. Eor this reason, atomic absorption requires a line source. 

Figure 10.6. Atomic absorption with A) a sharp-line source and (B) a spectral-continuum source. AA = absorption line half-width AA, = source line half-width S = spectral bandwidth of monochromator. Adapted from G. D. Christian and F. J. Feldman, Atomic Absorption Spectroscopy Applications in Agriculture, Biology, and Medicine, New York Wiley-Interscience, 1970, p 58, by permission of John Wiley and Sons.

Figure 10.12. Typical setup for atomic-absorption background correction with a continuum source.

Atomic fluorescence spectrometry has a number of potential advantages when compared to atomic absorption. The most important is the relative case with which several elements can be determined simultaneously. This arises from the non-directional nature of fluorescence emission, which enables separate hollow-cathode lamps or a continuum source providing suitable primary radiation to be grouped around a circular burner with one or more detectors. 

The photolysis of COS has proven to be a most valuable source of S atoms for kinetic studies93 and considerable effort has been spent in investigating the photochemistry of this molecule. Absorption commences at 2550 A with a continuum extending to 1600 A2 photolytic studies in this region93-95 have been confined to X > 2200 A where the primary processes... 

U. Heitmann, M. Schutz, H. Becker-Ross and S. Florek, Measurements on the Zeeman-splitting of analytical lines by means of a continuum source graphite furnace atomic absorption spectrometer with a linear charge coupled device array, Spectrochim. Acta Part B, 51, 1996, 1095-1105. 

Atomic Fluorescence Spectrometry. A spectroscopic technique related to some of the types mentioned above is atomic fluorescence spectrometry (AFS). Like atomic absorption spectrometry (AAS), AFS requires a light source separate from that of the heated flame cell. This can be provided, as in AAS, by individual (or multielement lamps), or by a continuum source such as xenon arc or by suitable lasers or combination of lasers and dyes. The laser is still pretty much in its infancy but it is likely that future development will cause the laser, and consequently the many spectroscopic instruments to which it can be adapted to, to become increasingly popular. Complete freedom of wavelength selection still remains a problem. Unlike AAS the light source in AFS is not in direct line with the optical path, and therefore, the radiation emitted is a result of excitation by the lamp or laser source. 

With the majority of food products, copper can usually be determined by FAAS. In the few instances of low copper levels, resort has commonly been made to chelation—solvent extraction prior to FAAS measurement [8d, 34, 37m, 217] recent reports on the application of EAAS to analysis of foodstuffs and biological materials have appeared [193, 201, 204, 205, 223, 224], The importance of non-atomic absorption in the determination of copper in plant digests by FAAS has been studied by Simmons [225]. Correction was necessary to arrive at good results, with a continuum light source providing more accurate data than use of a nearby non-absorbing line. 

One of the main practical problems with the use of AAS is the occurrence of molecular species that coincide with the atomic signal. One approach to remove this molecular absorbance is by the use of background correction methods. Several approaches are possible, but the most common is based on the use of a continuum source, D2. In the atomization cell (e.g. flame) absorption is possible from both atomic species and from molecular species (unwanted interference). By measuring the absorption that occurs from the radiation source (HCL) and comparing it with the absorbance that occurs from the continuum source (D2) a corrected absorption signal can be obtained. This is because the atomic species of interest absorb the specific radiation associated with the HCL source, whereas the absorption of radiation by the continuum source for the same atomic species will be negligible. 

Atomic fluorescence flame spectrometry is receiving increased attention as a potential tool for the trace analysis of inorganic ions. Studies to date have indicated that limits of detection comparable or superior to those currently obtainable with atomic absorption or flame emission methods are frequently possible for elements whose emission lines are in the ultraviolet. The use of a continuum source, such as the high-pressure xenon arc, has been successful, although the limits of detection obtainable are not usually as low as those obtained with intense line sources. However, the xenon source can be used for the analysis of several elements either individually or by scanning a portion of the spectruin. Only chemical interferences are of concern they appear to be qualitatively similar for both atomic absorption and atomic fluorescence. With the current development of better sources and investigations into devices other than flames for sample introduction, further improvements in atomic fluorescence spectroscopy are to be expected. 

Figure 6.4 Width of an atomic absorption line (Zn 213.9 nm line), greatly exaggerated, compared with the emission bandwidth from a continuum source such as a deuterium lamp.

Figure 76 Mode of operation of a continuum source background corrector. A and B Atomic absorption only C and D Atomic absorption with background absorption...

Fig. 2.3. Absorbance as a function of optical density for selected shock tube investigations employing OH electronic absorption spectrometry. The unmarked curve represents the semi-empirical relationship derived in Reference 37, evaluated at a pressure (5 1 atm) and temperature (1520 K) typical of recombination experiments in an argon diluent. Tlie curves labelled 6 1, 3 1 and 1 3 were empirically determined over a selected range of recombination pressures and temperatures for mixtures dilute in argon with those particular initial H2/O2 ratios (Reference 32). The curve identified by HJ (Reference 24) was empirically determined in a 1 % Hg-l % 02-98 % Ar mixture at 1300 K for a selected range of pressures. The cross-hatched area represents the approximate range of absorbances and optical densities observed with an atomic bismuth line source (Reference 41). Also shown are the line HH derived from photographic spectroscopy using instrumental definition of absorption line centres on a continuum (Reference 48), and a solid circle (beyond the range of the abscissa) denoting the photoelectric absorbance reported in Reference 47 for a continuum source at an optical density of 750 x 10" moles liter cm.

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