Hollow cathode lamp spectral radiance

Furthermore, the radiant density of the D2 lamp in a large part of the spectrum is fairly low. Hence, the procedure limits the number of analytical lines which can be used and the number of elements that can be determined. As the spectral radiance of the D2 lamp is generally low as compared with that of a hollow cathode lamp, the latter must be operated at a low radiant output (low current), which means that detector noise limitations and poor detection limits are soon encoun-terd. Finally, as work is carried out with two primary radiation sources, which are difficult to align as they have to pass through the same zone of the atom reservoir, this may lead to further systematic errors. 

Coherent forward scattering (CFS) atomic spectrometry is a multielement method. The instrumentation required is simple and consists of the same components as a Zeeman AAS system. As the spectra contain only some resonance lines, a spectrometer with just a low spectral resolution is required. The detection limits depend considerably on the primary source and on the atom reservoir used. When using a xenon lamp as the primary source, multielement determinations can be performed but the power of detection will be low as the spectral radiances are low as compared with those of a hollow cathode lamp. By using high-intensity laser sources the intensities of the signals and accordingly the power of detection can be considerably improved. Indeed, both Ip(k) and Iy(k) are proportional to Io(k). When furnaces are used as the atomizers typical detection limits in the case of a xenon arc are Cd 4, Pb 0.9, T11.5, Fe 2.5 and Zn 50 ng [309]. They are considerably higher than in furnace AAS. 

In principle, incandescent sources, such as quartz-halogen lamps and different types of arc lamps filled with deuterium or xenon may be used for HR-CS AAS. But the most critical parameter for the selection of a radiation source is its spectral radiance in the important far-UV spectral region down to 190nm. In order to compete with the detection limits of LS AAS the spectral radiance per picometer bandwidth increment of a CS should be at least one order of magnirnde higher than the narrow emission lines of hollow cathode lamps, since the geometrical conductance of a HR-CS AAS spectrometer is lower by the same order of magnitude. 

Figure 3.2 Wavelength-dependent spectral radiance of the xenon short-arc lamp measured in the hot-spot and at different distances from the cathode, in comparison to some selected emission lines of hollow cathode lamps...

The wavelength-dependent spectral radiance of the hot-spot xenon lamp is depicted in Figure 3.2 for different distances from the cathode in relation to the prominent emission lines of some hollow eathode lamps. In addition, a comparison of its spectral radiance to a commercial xenon lamp operating in diffuse mode as well as a conventional D2-lamp is shown in Figure 3.3. 

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