Microfabricated passive vapor preconcentrator/injector designed for microscale gas chromatography

Abstract

The design, fabrication, and preliminary testing of a micromachined-Si passive vapor preconcentrator/injector (μPPI) are described. Intended for incorporation in a gas chromatographic microsystem (μGC) for analyzing organic vapor mixtures, the μPPI captures vapors from the air at a known rate by means of passive diffusion (i.e., without pumping) and then desorbs the vapor sample thermally by means of an integrated heater and injects it downstream (with pumping). The μPPI chip comprises a 1.8 μL deep reactive-ion-etched (DRIE) Si cavity with a resistively heated membrane floor and a DRIE-Si cap containing >1500 parallel diffusion channels, each 54 × 54 × 200 μm. The cavity is packed with 750 μg of a commercial graphitized carbon adsorbent. Fluidic and heat-transfer modeling was used to guide the design process to ensure power-efficient sample transfer during thermal desorption. Experiments performed with toluene at concentrations of ∼1 ppm gave a constant sampling rate of 9.1 mL min⁻¹ for up to 30 min, which is within 2% of theoretical predictions and corresponds to a linear dynamic mass uptake range of ∼1 μg. The cavity membrane could be heated to 250 °C in 0.23 s with 1 W of applied power and, with 50 mL min⁻¹ of suction flow provided by a downstream pump, yielded >95% desorption/injection efficiency of toluene samples over an 8-fold range of captured mass.