Gas Chromatographic Microsystems: Design and Implementation of Improved Devices for Sample Capture and Detection

Abstract

The on-site analysis of airborne volatile and semi-volatile organic compounds (S/VOC) is critical to addressing various issues, including human exposure assessment, homeland security, disease diagnostics, and exposure limit establishment. This dissertation focuses on the development of two critical Si-microfabricated components of a microscale gas chromatograph (μGC) for the determination (i.e., identification and quantification) of S/VOCs: the preconcentrator and the detector. The first study concerned a recently invented optical microsensor called the micro-optofluidic ring resonator (μOFRR). Resonator structures comprised SiOx cylinders ~80 µm tall with 50-250 µm i.d. and walls ≤ 2 µm thick after partial release from the Si substrate. Optical resonances, excited in the wall by coupling a modulated laser signal via a proximal optical fiber taper, shift when vapors reversibly partition into a film coated on the inner μOFRR surface. Qualitative tests with a PDMS-coated μOFRR confirmed resonance shifts upon exposure to each of three VOCs. Similar tests using μOFRR devices coated with a functionalized monolayer-protected Au nanoparticle (MPN) or a functionalized Au nanorod also gave positive results. Implementation of the MPN-coated μOFRR as a detector for a conventional GC yielded low-ng level detection limits. The second study concerned a so-called microfabricated collector-injector (μCOIN). As conceived, the µCOIN would consist of two series-coupled devices, a micro passive preconcentrator (PP) and a micro progressively heated injector. Here we describe the design, construction, and characterization of the PP. Discrete devices (8 × 8 mm chips) were made from a micromachined silicon-on-insulator top layer and a glass bottom layer, joined by Si- Au eutectic bonding. The top layer has an array of 237 square apertures (47 × 47 m) distributed around the periphery of a 5.2-mm diameter circular region through which vapors diffuse at predictable rates. Two concentric annular cavities, separated by Si pillars and offset inwardly from the apertures, are packed with ~800 µg each of the graphitized carbon adsorbents Carbopack B (outer) and X (inner). Thin-film Pt heaters on the bottom substrate are used to thermally desorb captured S/VOCs, which are drawn by a downstream pump through the center of the device to a micro focuser and then injected to a bench scale GC for analysis. Test compounds included common solvents and chemical warfare agent simulants, which spanned a vapor pressure range of 0.033 to 1.1 kPa. Effective (diffusional) PP sampling rates ranging from 0.16 to 0.78 mL/min were observed for short-duration samples among the 15 test compounds. Agreement between observed and modeled sampling rates was generally within ±15%, with exceptions explicable by one or more factors. Sampling rates for two representative compounds, diethylmethylphosphonate and o-xylene, declined by only ~20-30% from 0.25 to 24 hr of continuous exposure at fixed, low concentrations of these compounds. For o-xylene, the sampling rate declined by only 8% over a ~2,300-fold conc. range at a fixed sampling period of 0.25 hr. A model was developed that could estimate decreases in sampling rates as a function adsorbent saturation and time. Desorption/transfer efficiencies were > 85% for all individual vapors (most > 95%) at 250-275 C in 60 sec at 5 mL/min. Sampling rates for mixtures of up to eight compounds matched those for the individual compounds. Possible design revisions for next-generation devices are presented.