Sharp line sources

The most commonly used sharp line source is the hollow cathode lamp. 

A continuous source can be used for atomic absorption, but since only the center part of the band of wavelengths passed by the slit will be absorbed (due to the sharp line nature of atomic absorption), sensitivity will be sacrificed, and the calibration curve will not be linear. This curvature is because even at high concentrations, only a portion of the radiation passing through the slit will be absorbed, and the limiting absorbance will approach a finite value rather than infinity. With a sharp line source, the entire width of the source radiation is absorbed and so the absorption follows Beer s law. A continuous source works best with the alkali metals because their absorption lines are broader than for most other elements. Specificity is not as great with a continuous source because nearby absorbing lines or molecular absorption bands will absorb part of the source. 

Profiles of an absorption line, an emission line from a sharp line source and the bandpass of a monochromator. 

The instrumentation required for atomic fluorescence measurements is simpler than that used for absorption. As the detector is placed so as to avoid receiving radiation directly from the lamp, it is not strictly necessary to use a sharp-line source or a monochromator. Furthermore, fluorescence intensities are directly related to the intensity of the primary radiation so that detection limits can be improved by employing a high-intensity discharge lamp. 

As Ip depends on f, a stable, intense sharp-line source greatly enhances AFS sensitivity. Similarly, the geometry of the atom cell is important. 

It should be pointed out here that wavelength selection in atomic absorption spectroscopy is largely accomplished by the choice of the monochromatic sharp line source, possessing the wavelength of a resonance line of the element to be determined, a specificity of selection unobtainable by any other means. Any additional wavelength selection can be considered merely secondary and the methods to this end should be examined with this in mind. 

A sharp-line source is used in A AS. The source emits the lines of the element to be measured. These possess the precise energies required for absorption by the analyte atoms. 

Sources. A sharp-line source is required in atomic absorption because the width of the absorption line is very narrow, a few thousandths to one-hundredths of a nanometer, at most. Because the absorption line is so narrow, only a small fraction of the radiation from a continuum source passed by the slit and reaching the detector would be absorbed. 

Why is a sharp-line source desirable for atomic absorption spectroscopy ... 

With the continuous source, it is possible to produce analyses for a number of elements. However, the complexity of the required instrumentation, and the necessary operator skill, are far higher than are needed when working with sharp-line sources. Also, there are far higher risks of encountering spectral interferences, while sensitivity values are much lower. Largely for these reasons, there has not yet been any commercial development of continuous source atomic absorption equipment. 

A sharp-line source and a continuum source are compared in Figure 10.6. Figure 10.6A illustrates the absorption of light from a line source. The source line and the absorption line are at the same wavelength, but the half-width of the source (AAg) is narrower than that of the absorption line (AA ) because temperatures and pressures are lower in the source than in the flame. Therefore, the entire center of the source line is absorbed in accordance with Beer s law. 

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 10.7. Comparison of absorption from continuum and sharp-line sources.

A typical setup is shown in Figure 10.12. The two light beams from the lamps are combined by a half-coated mirror (e.g., coated with small reflecting circles so that it will reflect the continuum radiation but will allow space in the mirror for the radiation from the hollow-cathode lamp to pass). Each lamp is pulsed electronically to provide an AC signal, but the two lamps are 180° out of phase. A phase-sensitive detection system then measures the difference of the two signal intensities (which are initially balanced). The sharp-line source measures both atomic absorption and... 

Why will a nonlinear calibration curve and a loss in sensitivity generally occur in atomic-absorption spectroscopy when using a continuum light-source as compared to a sharp-line source ... 

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