Background absorption excessive

Here, B is the background absorption in the absence of chemical relaxation, ca the angular frequency of the inflection point of the absorption ca = 2nf (rad s ) and X the relaxation time (s). The excess absorption due to chemical relaxation A is obtained from the low-frequency absorption where ca x and is given by ... 

As, Se, Pb, and many more would have been lost in the process. By adding an excess of ammonium nitrate as a matrix modifier to the sample in the graphite tube, the NaCl matrix can be converted to the much more volatile compounds ammonium chloride and sodium nitrate. Most of the matrix can be removed at a temperature below 500°C, which substantially reduces background absorption without loss of volatile analytes. 

Rising above the background emission, assumed to be that of a blackbody at 3000°K, is a broad 9.7-jam feature for the oxygen star. Between about 15 and 20 jim a much weaker excess emission feature is also evident. A measured absorption spectrum of amorphous olivine smoke particles (Kratschmer and Huffman, 1979) is shown for comparison. 

Spectral interferences are uncommon in AAS owing to the selectivity of the technique. However, some interferences may occur, e.g. the resonance line of Cu occurs at 324.754 nm and has a line coincidence from Eu at 324.753 nm. Unless the Eu is 1000 times in excess, however, it is unlikely to cause any problems for Cu determination. In addition to atomic spectral overlap, molecular band absorption can cause problems, e.g. calcium hydroxide has an absorption band on the Ba wavelength of 553.55 nm while Pb at 217.0nm has molecular absorption from NaCl. Molecular band absorption can be corrected for using background correction techniques (see p. 174). The operation of a flame atomic absorption spectrometer is described in Box 27.6. 

Flame atomic absorption is subject to many of the same chemical and physical interferences as flame atomic emission (see Section 28C-2). Spectral interferences by elements that absorb at the analyte wavelength are rare in AA. Molecular constituents and radiation scattering can cause interferences, however. These are often corrected by the background correction. schemes discussed in Seetion 28D-2. In some cases, if the source of interference is known, an excess of the interferent can be added to both the sample and the standards. The added substance is sometimes called a radiation buffer. 

When very thin layers are measured (q)proximately 1 pm), reflectance effects are rarely a problem. Furthermore, excessive temperature gradients are also avoided by using very thin samples, thus reducing the re-adsorption of intense frequencies by cooler, outer layers. As die sample thickness is increased past a certain point, the spectral contrast between an actual band and the background noise decreases. Both reflectance and re-absorption effects result in attenuation of the stronger bands. Thus, the best spectra are obtained with the thinnest samples. 

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