Kinetic Factors Affecting the Design and Operation of Micropreconcentrators for mGC Instrumentation

Thitiporn Sukaew and Edward T. Zellers

Photograph of cPCF (top) and µPCF (bottom)

The goal of this project is to investigate the effects of kinetic factors on the dynamic adsorption capacity of an adsorbent-packed microfabricated preconcentrator/focuser (µPCF) for a microscale gas chromatograph (µGC). The single-stage deep-reactive-ion-etched Si cavity of the mPCF is packed with ~2 mg of a commercial graphitized carbon adsorbent with a high specific surface area. The efficiency with which vapor phase analytes are captured is a function of the thermodynamic equilibrium adsorption capacity and the rate at which vapors diffuse into the macropores of the adsorbent. The shorter the bed residence time (t) of vapors in the device, the more important is the diffusion rate. The critical bed residence time, tc, corresponds to the point at which breakthrough is observed immediately. By measuring the fractional breakthrough time of a vapor in a test atmosphere passing through the device versus t, one can evaluate tc and also establish limits on the allowable flow rate at which operation will yieldquantitative capture. For example, tests performed with four vapors spanning a vapor pressure range of 25-95 mm Hg, tc ranges from ?highest? to ?lowest? msec (10-170 mL/min) and varies inversely with analyte vapor pressure. Testing with a similarly packed capillary style device (cPCF) suggests a small but measurable effect arising from the adsorbent-bed geometry. Results of these tests are being used to establish constraints on the operating parameters of mGC prototypes being developed for explosives detection, breath biomarker measurements, and indoor air quality monitoring. This project is supported by the Department of Homeland Security, Science and Technology Directorate.