Although many analytical methods are available for the determination of elements when they occur alone or in simple combination, such conditions are rarely met in practice. Instead, technological development gives rise to continuing and increasing demands for more rapid and more accurate methods of analysis, especially for the determination of traces of one or more elements in multicomponent mixtures. At the same time, there is the further requirement for simplification, as far as possible, of the analytical procedure. These needs are being met in three main ways: Older methods are being refined, new analytical procedures are being devised, and physical methods of measurement, such as atomic absorption spectrometry, are being used to an ever greater extent. Even in the last group, however, practical chemical treatments such as preliminary separations or concentrations are frequently necessary. For example, the atomic absorptiometric determination of chromium in ashed samples of whole blood requires a separation step if interference from the relatively high concentration of iron in the sample is to be overcome.