Micro-Scale Preconcentrators for Vapor-Phase Air Contaminants: Optimizing the Design and Operating Conditions for Integration with Micro-Scale Gas Chromatographic Instrumentation.

Abstract

The broad goal of this research was to improve the design of miniaturized and microfabricated vapor preconcentrator devices used to capture, focus, and thermally inject analytes in microfabricated gas chromatographs (µGC) for the determination of toxic vapors encountered in workplace or residential environments. Microfabricated preconcentrator/focusers (µPCFs) with different sized cavities and capillary preconcentrators (cPCF) packed with ~ 1-2.4 mg of a graphitized carbon, Carbopack X, were challenged with low concentration of 2-butanone, benzene, n-heptane and toluene over a range of flow rates. Carbon nanotubes (CNTs) were tested in similar manner. The breakthrough time, tb, decreased monotonically with decreasing bed mass and bed residence time, τ, for all vapors, as described by the modified Wheeler model. The performance of the largest μPCF was comparable to that of a reference cPCF with a similar bed mass. The critical flow rates (Qc) ranged from 70-290 mL/min and varied inversely with kinetic equilibrium adsorption capacity (We-k). As a practical operating guideline, it is recommended that flow rates be limited to less than 0.4Qc for reliable performance. The breakthrough time ( tb) of CNTs from both types of devices follow the model description, but the difference between the adsorbent packing densities prohibits dynamic adsorption capacity comparisons of the two materials. The results from kinetic rate coefficient (kv) study showed minimal contribution of vapor characteristics to this parameter, and allow the development of single kv prediction model for all vapors. The modified Wheeler model performance in predicting the tb of binary mixtures was investigated. The underestimation of the equilibrium adsorption capacity parameter in a binary mixture by the Molar Proportionality Model resulted in the underestimation of tb, therefore it is not recommended to extend the single-vapor results to mixture analysis. A multi-stage preconcentrator/focuser (PCF) module for a portable µGC prototype, consists of a pre-trap, high-volume sampler, and microfocuser was developed and characterized for the specific problem of quantifying extremely low air concentrations of trichloroethylene (TCE) in homes at risk of vapor intrusion from underlying contaminated groundwater. It provides preconcentration factor as high as 800,000.