The goal of this project is to investigate the effects of key design and operating variables on the dynamic adsorption capacity of adsorbent-packed microfabricated preconcentrator/focusers (µPCF) used in microscale gas chromatographic analyzers (µGC). The µPCFs under consideration comprise several single-stage deep-reactive-ion-etched Si cavities packed with a few mg of graphitized carbon adsorbent materials with high specific surface areas. The modified Wheeler model, which relates thermodynamic and kinetic parameters to the vapor capture efficiency, is used to interpret the effects of changes in cavity size, flow rate, vapor concentration, and vapor properties on the breakthrough time tB and breakthrough volume VB of the µPCF. By measuring the fractional breakthrough concentration of a vapor in a test atmosphere passing through the device versus time, one can establish limits on the allowable flow rate at which a device of given dimensions will yield quantitative capture. Tests performed with four vapors spanning a vapor pressure range of 25-95 Torr revealed that immediate breakthrough occurs if the flow rate (Q) exceeds 50 mL/min for the most volatile vapor and 230 mL/min for the least volatile vapor. Testing with a similarly packed capillary style device (cPCF) suggests a small but measurable effect arising from the adsorbent-bed geometry differences between the cPCF and µPCF. Results of these tests are being used to establish constraints on the operating parameters of µGC prototypes that employ such devices, which are being developed for explosives detection, breath biomarker measurements, and indoor air quality monitoring. This project was supported by the Department of Defense Environmental Security Technology Certification Program (DoD-ESTCP).