The future of plasma spectrochemical instrumentation. Plenary lecture
Every decade seems to bring with it some important novelty in atomic spectrochemical instrumentation. During the past decade, however, the changes seemed to be more evolutionary than revolutionary. Emission spectrometric instrumentation has become less expensive and more capable with the introduction of advanced user interfaces, lowered detection limits that now approach those for electrothermal atomization in AAS, axial-viewing options that provide greater stability and higher sensitivity but at the cost of elevated interferences, and multichannel detector arrays that enable an entire emission spectrum to be viewed at once. Similarly, MS instrumentation has evolved to a simpler, less expensive form even as capabilities have increased. In atomic MS, the origins of several troublesome interference effects have been identified and substantially reduced, and strategies have been devised to reduce the severity of isobaric overlaps (spectral interferences). Because of these trends, sales of emission-based instrumentation have remained brisk, while those for atomic mass spectrometers have risen dramatically. The combination of relatively high sales and higher capabilities has encouraged a number of new instrument manufacturers to enter the plasma spectrochemical market, while others have been forced to drop out, to consolidate or to be acquired. In attempting to project the future of atomic spectrochemical instrumentation, it is a safe bet to assume that past trends will continue. Revolutionary changes are, of course, much more difficult to forecast. Nevertheless, some guidance can be derived from reviewing the limitations of current sources and detection techniques that are used for plasma spectrometry. Such a review reveals that detection limits in emission measurements are usually constrained by background noise levels, whereas those in MS are bounded by the efficiency of sample utilization and by the transmission of the mass spectrometer and of the interface that separates it from the plasma source. Also, although the origin of matrix interferences in both atomic emission and atomic mass spectrometry are not fully understood, it seems clear that what is needed is better control of sample introduction, atom formation and the plasma environment that fosters atomic excitation and ionization. Further, it is obvious that more information must be derived from each atomic spectrometric measurement, in order to learn more about sample speciation and to enable the instrument better to monitor its own operation. These needs and likely trends argue strongly for a higher degree of dimensionality in atomic spectrometric measurements. Higher dimensionality, represented in other areas of analytical chemistry by the so-called ‘hyphenated techniques’ such as GC–MS, can be achieved in atomic spectrometry by using sources and sample-introduction techniques in tandem (either in series or in parallel), by combining emission, MS and AF measurements and by employing multi-dimensional calibration and sample-recognition algorithms. For example, it can be shown that MS resolution greater than 300 000 can be achieved by means of a relatively simple, moderate-resolution mass spectrometer, as long as it is preceded by suitable sample-introduction apparatus. Further, interference effects and sample-utilization efficiency might be dramatically increased by introducing sample solutions in the form of discrete droplets or as puffs of sample vapour. These and other examples, taken from the author's laboratory and from laboratories of others, illustrate these various trends and future projections.