CRITICAL COMPARISON OF SOURCES AND DETECTION METHODS IN ATOMIC SPECTROMETRY

Over the past three decades, the nature of atomic spectrometric analysis has changed dramatically. In the 1960's, flame and furnace atomic absorption were dominant, with furnace techniques gradually achieving prominence. In contrast, the 1970's witnessed the introduction of the inductively coupled plasma (ICP) and the direct current plasma (DCP). Although the ICP is still the most frequently used excitation source, the decade of the 1980's has seen a resurgence of interest in the glow-discharge lamp (GDL) and the microwave-induced plasma (MIP). Today, novel sources are continually being introduced, supported by combinations of gas mixtures and operated alternatively at DC, microwave or radio frequencies. These advances must be assessed against the backdrop of more unconventional atom-formation techniques that involve the use of high-powered lasers, rare-gas sputtering systems, or tandem (coupled) sources. In a similar vein, modes of detection have changed significantly over the past 30 years. While chemical flames and furnaces were the atom reservoirs of choice, atomic absorption was favored, in large measure because of its simplicity. In contrast, in the period of the 1970's, the attributes of the ICP made emission spectrometry the most attractive, in large part because of its straightforward adaptation to simultaneous multielement approaches. The most common configurations for such emission-based measurements were the direct-reading spectrometer, borrowed from earlier arc and spark days, and the slew/scan arrangement, in which desired spectral regions were scanned slowly, but regions of less interest bypassed hastily. In both systems, it was recognized that background correction was becoming increasingly important. In the 1980's, we have come to realize that other detection techniques might ultimately prove superior to a conventional emission measurement. For example, plasma-source mass spectrometry has already demonstrated detection limits several orders of magnitude lower than can be achieved routinely by emission. Similarly, atomic or ionic fluorescence, excited by a laser source, has been shown capable of detecting as little as a few thousand atoms. Ultimately, techniques such as resonance ionization spectrometry are appealing, despite their complexity, since they are capable of detecting even a single atom. Adding to this rather complicated situation is the question of how emission might best be detected if it is the measurement of choice. The traditional direct-reading and slew/scan approaches are convenient and well established. However, they lack the ability to detect a complete atomic-emission spectrum, a feature offered long ago by a photographic film or plate. Now, such capability exists in multichannel electronic readout devices, of either the linear or two-dimensional variety. The advent of charge-coupled devices (CCD), charge-injection devices (CID) and integrated linear photodiode array packages (PDA) makes it possible to consider replacing single-channel detectors with an electronic analog of the photographic plate. Similarly, the use of Fourier-transform spectrometry, produced by means of a moderate-resolution interferometer, affords the option not only of simultaneous multielement analysis, but also of simultaneous monitoring of background levels, accurate registration of atomic-emission wavelengths, and relatively rapid scan speeds. In this paper, a few of the more attractive alternatives in this apparently bewildering array of options will be outlined. The most attractive of the options will be compared with each other in an attempt to assess not only what the most attractive current choices are in atomic spectrometric analysis but also what the future might bring. © 1990 Springer-Verlag.