EDITORIAL
Ode to the Separator This column is another in the series on Frontiers in Analytical Chemistry. Some readers may have wondered by my choice of previous topics whether I see no frontiers in the more traditional analytical specialty areas such as separations science, spectroscopy, electroanalytical chemistry, etc. Far from it! Here I present some thoughts about separating things. A longstanding goal of chemical analysis is specificity, or response of the measurement to a particular chemical species. Nirvana is the availability of as many completely selective chemical sensors as there are interesting chemicals. Remarkable progress has been made in chemical selectivity, but reckoned against the enormous diversity of complex analyte mixtures, analytical chemistry's progress in direct specificity of chemical measurements has been distressingly slow. Part of this failing may be attributable to the content of analytical chemistry courses. But equally potent is the power of modern separation science methods that can separate-in both time and space-complex mixtures of chemical species with subtle differences. Separation science has lessened some of the urgencies in attaining chemical specificity in analytical measurements of complex mixtures. It has in the meantime saved much bacon for analytical chemists. A successful chemical separation requires specificity of interaction of the chromatographic column or electrophoretic field with the components of a mixture to produce the mixture-resolving differential migration rates. The degree of chemical specificity t h a t is needed can be extremely small-far below the degree required in a chemical sensor exposed to unresolved mixtures. Nonetheless, improving chemical selectivity remains a vital frontier of separations science. A better understanding of weak chem-
ical specificities is important. For example, the interaction between a sample molecule and a bonded monolayer of stationary phase is readily measured as a capacity factor, but this gives little insight into the actual chemical reactivity and associated structural details of that interaction. It's like trying to understand acid-base chemistry solely from pK, values. Some efforts have been made to explicitly probe reactivity and structural details, and this is an important basic research direction. Mother Nature's specificity can also be invoked in designing chemical separations in which the sample species is known and targeted, as in affinity columns. This is another important direction. The other crucial frontier is in detector sensitivity. The detectability of zones containing small quantities of material, owing to dilute samples or small sample sizes, is frequently a limitation in mixture analysis. Laser-induced fluorescence detection has produced significant advances in observations of low concentration zones, and in CZE, where the sample size is also small, the advances are remarkable in terms of moles measurable. Detection of sample zones containing only - 10,000 molecules has been achieved with the use of laser-induced fluorescence detection in CZE. Such zeptomole capability, along with the separating power of CZE, brings closer to reality the investigation of problems requiring mixture analysis of extremely small spaces, such as single biological cells, lithographic patterns, and interfaces. While achieving chemical specificity deserves continued priority in analytical research, the separation scientists among us deserve the admiration of all for leadership in solving problems of complex mixture analysis.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 13, JULY 1, 1992
661 A
