Design and Characterization of a Multi-Vapor Preconcentrator for a Micro-Scale Gas Chromatograph.

Abstract

This dissertation addresses several elements critical to the design and implementation of adsorbent-packed micro-scale preconcentrator/focuser (PCF) intended for integration into a microfabricated gas chromatograph (GC). Assessment of the adsorption of volatile organic compounds (VOCs) on two commercial graphitized carbons at air concentrations in the parts-per-billion (ppb) range in the context of the Dubinin-Radushkevich (DR) isotherm model revealed an unusual bi-modal distribution of characteristic adsorption energies for most vapors. Systematic trends in observed VOC behaviors on these non-microporous adsorbents are rationalized in terms of the volatility and polarity of the adsorbates, and the relative strength of adsorbate-absorbent interactions over different ranges of surface coverage. The attempt to apply the DR model at such low VOC concentrations is unprecedented, and the results highlight several constraints on its applicability for estimating the mass of adsorbent required for efficient PCF performance. The effect of flow velocity on the capacity of an adsorbent-packed PCF containing ~1 mg of adsorbent was characterized along with a similarly packed device of a more conventional design. It was shown for two representative vapors that below a bed residence time of 4 msec, corresponding to volumetric flow rates of 30 mL/min, fractional breakthrough occurred almost immediately. The critical bed residence time differed between the vapors in a predictable manner, and guidelines were established on the basis of these results for limiting the operational flow rates of PCFs. Using the conventional PCF a series of tests was then performed to explore the capacity of the device upon exposure to binary mixtures comprising vapors with different volatility and structure. Comparison of the results with those expected on the basis of recently developed models derived from larger adsorbent beds revealed significant discrepancies, which suggests that improvements to such models are needed; however several factors were revealed that may facilitate future efforts to improve the models. These first attempts to verify the application of established thermodynamic and kinetic models of adsorption to PCFs containing low-mg quantities of adsorbents have produced important results critical to their design and implementation in microanalytical systems for VOC mixtures.