A Micro Passive Preconcentrator For Micro Gas Chromatography

We describe a microfabricated passive preconcentrator (mu PP) intended for integration into gas chromatographic microsystems (mu GC) for analyzing volatile/semi-volatile organic compounds (S/VOC). Devices (8 x 8 mm) were made from a silicon-on-insulator top layer and a glass bottom layer. The top layer has 237 apertures (47 x 47 mu m) distributed around the periphery of a circular region (5.2 mm o.d.) through which ambient vapors diffuse at predictable rates. Two internal annular cavities offset from the apertures are packed with similar to 800 mu g each of commercial carbon adsorbents. Thin-film heaters thermally desorb captured vapors, which are drawn by a pump through a central exit port to a micro injector for analysis with a bench scale GC. The 15 test compounds spanned a vapor pressure range of 0.033 to 1.1 kPa. Effective (diffusional) mu PP sampling rates ranged from 0.16 to 0.78 mL min(-1) for short-duration exposures to similar to mg m(-3) vapor concentrations. Observed and modeled sampling rates generally agreed within 15%. Sampling rates for two representative compounds declined by <= 30% between 0.25 and 24 h of continuous exposure. For one of these, the sampling rate declined by only 8% over a similar to 2300-fold concentration range (0.25 h samples). Desorption (transfer) efficiencies were >95% for most compounds (250-275 degrees C, 60 s, 5 mL min(-1)). Sampling rates for mixtures matched those for the individual compounds. Dissipating no energy while sampling, additional advantages of this novel device include short- or long-term sampling, high capacity and transfer efficiency for a diverse set of S/VOCs, low transfer flow rate, and a robust fabrication process.