Chemical Sensors and Fast Separations
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n last month's editorial I spoke about the challenges of achieving molecular selectivity in the responses of chemical sensors. There is a steady drive to bring analytical chemistry into the field; examples include control of chemical processes, environmental monitoring, chemical weapons treaty verification, consumer health diagnostics, and atmospheric and oceanographic studies. The term chemical sensor is applied to an analytical device designed for field or plant use. Solid-state electrochemical cells for dioxygen monitoring in automobile engines are perhaps the most widely used chemical sensors. There are many other examples of successful sensors, but the needs list for field measurements of molecular analytes seems to be both long and growing in relation to the success stories of molecularly selective sensors. In the fully equipped analytical laboratory, molecular selectivity in a complex sample matrix is achieved by frontal assault. The sample matrix is chromatographically or electrophoretically separated into its different components, which are then detected as pure substances by devices that can lack molecular selectivity because they don't need it, and structurally characterized as pure substances by vibrational spectroscopy or MS. A single analytical system has the power to detect and determine a large range of molecular constituents. Wouldn't it be great to bring that power into the field? The idea of a separation system as a chemical sensor has been actively pursued by several groups in recent years. The important elements are as follows: The chemical separation must be fast, so that the overall response can compete with the response times of direct-reading, selective sensors. Examples exist of sec-
onds-scale gas and liquid chromatographic separations and of subsecond capillary electrophoresis. The separations resolution is degraded on fast time scales but can still be adequate for many applications.The separation apparatus must be miniaturized and "ruggedized," for reasons of portability, cost, and reliability. Exciting directions in miniaturization include lithographically defined chromatographic columns and electrophoretic channels made by micromachining. Sample injection can furthermore be automated in these formats, and a variety of detection devices are available. An important missing element thus far is a device such as a miniaturized mass spectrometer to measure structures of separated components, which will frequently be necessary. A discussion of the limitations that physical principles might place on mass spectrometer miniaturization would be a welcome addition to the literature. As in fast separations, some degradation in mass resolution performance may be acceptable for many applications.The supporting electronics must also be simplified and miniaturized, which may be a serious problem relative to direct-reading chemical sensors. Although much research remains to be done to prove, by practical applications, the merits of separations-based chemical sensors, I am optimistic about this area. Separations-based chemical sensors represent a frontier of analytical chemistry that has appeared as a bright light on the horizon and that deserves a wider range of involvement from the analytical research community.
Analytical Chemistry, Vol. 66, No. 1 1, June 1, 1994 625 A
