ANALYTICAL MEASUREMENT IN ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROMETRY - HOW CORRECT IS IT

The detection system for atomic absorption spectrometers based on the use of photomultiplier tubes (PMTs) is analysed from the point of view of its ability to provide accurate analytical information. It is show n that absorbance recorded by the system depends not only on the numbe r of absorbing atoms but also on their distribution in the furnace vol ume. The typical non-uniformity of atomic distribution that occurs in graphite furnaces and its impact on the recorded signal are discussed. The cross-sectional distribution of the intensity of the radiation be am from a primary source was measured at different locations of the sp ectrometer for different source operating conditions. The distribution is rather non-uniform and can be described by the Gaussian function. An analysis of the joint effect of the radiation and analyte non-unifo rmity on the absorbance measured is given. The shape of the radiation beam cross-section changes from a circle to an ellipse with increasing lamp current. A new. detection system based on the use of a solid-sta te detector (photodiode array, charge coupled device, charge injection device) instead of PMTs is proposed. The solid-state detector is loca ted vertically along the monochromator exit slit and allows the detect ion of spatially resolved absorbances. It is shown that the analytical signal recorded by this new system is proportional to the number of a bsorbing atoms irrespective of the non-uniformities described above.