Heated graphite atomization systems for atomic fluorescence spectrometry



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In this thesis four investigations undertaken in the development, lifetime, efficiency and application of heated graphite atomization systems are presented. The first investigation involves a comparison of four different atomization systems having little or no background emission. The comparison is made using an atomic fluorescence system employing combinations of three different types of primary excitation sources and two different types of signal processing systems. This investigation resulted in the development of a graphite tube atomization system which combines the best qualities of the three other atomization systems. The original graphite tube atomization system was modified for a second investigation to improve its efficiency and stability. In this investigation the graphite tube system is used in conjunction with a low intensity continuum source, photon counting electronics and a small monochromator for atomic fluorescence spectrometry. Detection limits with this system are determined for 13 elements. For most of these elements the detection limits are comparable to those obtined by conventional atomic absorption spectrometry employing flame atomization and individual line sources for each element. The third investigation is the use of a pyrolysis treatment for graphite atomization systems. By introducing a mixture of methane and an inert gas into a graphite atomization system, and operating the heated element at temperatures above about 2000[degrees]C, a layer of pyrolytic graphite is deposited on the graphite element surfaces. This layer is very dense, hard, impermeable to gases, non-porous and resistant to oxidation. For graphite atomization systems operated continuously at high temperatures, the gradual deterioration of the heated element can be eliminated completely by the addition of a small amount of methane so that the lifetime of the element becomes relatively indefinite. The final investigation is a direct application of information obtained from the previous investigations. An atomic fluorescence system is designed which consists of the graphite tube atomization system, a high intensity electrodeless discharge lamp, a high aperture monochromator and phase sensitive electronics. Using this system microsamples of the enzyme RNA polymerase (MW 470,000) are analyzed for zinc content. The results of this analysis show the mean zinc content of the enzyme to be 1.99 [plus-minus] 0.03 zinc atoms per molecule. The determination requires the use of zinc concentrations very near or below the detection limit of conventional atomic absorption instruments, while the detection limit for the atomic fluorescence system is at least three orders lower.