Analytical Chemistry in Small Spaces Analytical chemistry is pushing the frontiers of its capabilities in all directions. One direction is the measurement of chemical systems in small spaces such as the picoliter-sized volumes of biological cells and the active regions of capillary zone electrophoresis fluorescence detectors; the femtoliter-sized volumes of 1-pm aerosol droplets (e.g., those generated in atmospheric fog or electrospray mass spectroscopy) and of 1-ym particles (e.g., those troubling to microelectronics processing); and the attoliter - sized volumes of 100-nm quantum particles and square microns of receptors or channel-containing lipid bilayer membranes. There are many small-space situations, and learning how to conduct chemical measurements on and in them is of both fundamental interest and great practical importance. Taking measurements in small spaces can have a plethora of objectives. Simply determining the size of the space can be one. Others include determining charge or mass, or obtaining an electronic fluorescence spectrum. With increasing difficulty, we may wish to inquire about the presence of a particular element or molecule, or we may seek a fuller description of the quantities or the variety of elements or molecules present. One type of small-space measurement may be detection for a separations experiment; the extreme resolution and small sample sizes of capillary zone electrophoresis will provide detector challenges for years to come. Knowing the homogeneity of composition of the small space can be important, such as in a biological cell, where an average analysis of concentration
obscures the fact that many intracellular components are packaged into even smaller, intercellular spaces. And the interest may not be in measurement of the value of a time-varying, dynamic, small-space quantity, but, rather, a static one. The methodology required for chemical measurements in small spaces is extremely demanding, not only of sensitivity but also of spatial selectivity. A number of significant technical advances offer spatial selectivity; these include optical fiber spectroscopy, electrodes of submicron dimensions, capillary electrophoresis, and focused lasers as microprobes-all exciting tools to have. More are needed, however, as is an improved ability to fabricate small devices and probes. Additionally, there are needs for "model" smallspace chemical systems upon which to cut the teeth of current and new tools, and for techniques suitable for selectively delivering chemical reagents to microscopic spaces or for organizing them therein. Dealing with the challenges of measurements in small spaces is yet another of the frontier areas of analytical chemistry discussed in this series of EDITORIALS. It is a frontier that lies beyond the still-current frontier of analysis of extremely small concentrations. Measurement science scholars and practitioners in academe and industry alike have strong reasons to confront the challenge of small-space measurements.
ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, 1991
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