Continuous background absorption

Figure 7.26 Fano profiles for q < 0, q = 0, q > 0, and q = oo (and —oo). The dotted line corresponds to the continuous background absorption (

Next, a certain number of analyte intensity scans is recorded, giving the three-dimensional plot, intensity versus wavelength and time, shown in Figure 5.3. In total, three different types of absorption signals can be observed continuous background absorption, analyte absorption, and fine-structured atomic or molecular absorption from concomitants. 

There are two possibilities to eliminate the continuous background absorption part. The first is based on the selection of so-called background correction pixels (BCP). The intensities of these particular pixels are summed and used for the correction by calculating the ratio of the sum intensities (/bcp and /Bcpscan) of the BCP in the averaged reference spectrum 7 (refer to Figure 5.2) and in each individual analyte intensity spectrum respectively. This gives a correction factor ascan for the individual scan ... 

This intensity correction factor is a measure of the continuous background absorption, leading to a decrease in the total signal intensity within the recorded spectral interval (see Figure 5.3) and is quite similar to the non-specific background value provided by conventional LS AAS systems, as shown in Figure 5.4. 

Figure 5.4 Temporal behavior of the intensity conection factor as a measure of the continuous background absorption...

In this way, continuous background absorption is removed and, in addition, all systematic errors, such as pixel errors and intensity fluctuations of the continuum source, are also eliminated, resulting in the three-dimensional plot of absorbance versus wavelength and time, shown in Figure 5.6, where only atomic and flne-structured absorption remains. 

With this technique, problems may arise with interference, such as background absorption—the nonspecific attenuation of radiation at the analyte wavelength caused by matrix components. To compensate for background absorption, correction techniques such as a continuous light source (D2-lamp) or the Zeeman or Smith-Hieftje method should be used. Enhanced matrix removal due to matrix modification may reduce background absorption. Nonspectral interference occurs when components of the sample matrix alter the vaporization behavior of the particles that contain the analyte. To compensate for this kind of interference, the method of standard addition can be used. Enhanced matrix removal by matrix modification or the use of a L vov platform can also reduce nonspectral interferences. Hollow cathode lamps are used for As, Cu, Cr, Ni, Pb, and Zn single-element lamps are preferred, but multielement lamps may be used if no spectral interference occurs. 

FIGURE 5.4 An x-ray spectrum from a normal skin sample obtained by PIXE-analysis. Detectable peaks are seen above the continuous background (Bremsstrahlung) radiation. Note that the Na peak is not detectable (due to absorption in the sample and the detector window). 

When determining those elements which absorb in the UV portion of the spectrum, corrections for nonatomic absorption are almost essential. This is especially true for solutions of high solids content, such as those which result from fusion techniques. This correction is generally made by using a hydrogen continuum lamp. Some of the newer instruments have provisions for automatic and continuous background correction. A field installable kit is available to retrofit the model 1200 now used. 

Here, fj.o(k) corresponds to the atomic absorption cross section as a function of k [cf. Eq. (6)]. This quantity represents the continuous background of the EXAFS-signal (cf. Fig. 1), whereas p,(k) is due to the oscillatory part of the signal. The EXAFS-equation is given in Eq. (11) ... 

Although many methods have been used to establish the temperature of a flame, the most widely accepted technique is the line reversal method. The flame is heavily doped with an element, usually sodium, which has a conveniently placed and easily excited resonance line. This resonance line is then viewed by spectroscope against the continuous background of a lamp, whose operating temperature is adjusted until the resonance lines disappear. If the flame is hotter than the lamp, the lines appear in emission (bright) while if the lamp is the hotter, the lines appear in absorption (dark). When the lines are not visible, the lamp and flame are believed to be at the same temperature. The temperature of the lamp filament is then measured independently with a pyrometer. The technique is discussed in detail, and the necessary precautions are outlined by Wolfhard and Gaydon or Lawton and Weinberg. ... 

The use of a continuous radiation source can also eliminate background absorption. In this method, radiation from a hollow cathode lamp and... 

The spectrometer can be equipped for quasi-simultaneous measurement of the line and background absorption [159]. Radiation from a second, continuous source, such as a deuterium arc lamp, is rapidly switched with the radiation of the primary source and fed through the ab.sorption volume, by a semitransparent mirror. The radiant flux of the continuous source is not decreased significantly by atomic absorption, owing to the low spectral resolution of the monochromator however, it will be weakened by broad-band absorption of molecules, or stray radiation. By this means, nonelement-specific background absorption can... 

---