Molecular background in flame AAS

In Chapter 7 several electron excitation spectra of diatomic molecules that have been recorded in a flame were shown. Some of these spectra are only observed in the presence of specific mafrices or acids, such as those of the PO and CS molecules, whereas others, particularly ihe spectrum of OH, is present in any spechoscopic flame, even when no sample is introduced. The concentrations of radicals such as O, OH, CN and H in a flame are determined by the reactions between the natural components of the flame. The influence of any sample constituents on these components is negligible, since every reaction that leads to a reduction or an increase in the concentration of such a species is immediately counteracted by infinitesimal shifts in the equilibria of the main components. This means that these molecular absorption structures in a flame should remain constant, and be eliminated in the calibration process. However, this is not exactly the case, as will be shown in the following examples. 

As in the case of lead the atomic line does not overlap with any molecular structure, no improvement in the LOD is obtained by the use of least-squares BC. However, it should be mentioned that additional noise, and even measurement errors, could be introduced in LS A AS when deuterium background correction is used in this case. 

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