A microfabricated preconcentrator for sample extraction and injection in microscale gas chromatography.

This thesis details the development of a microfabricated preconcentrator that functions as an extractor-injector for a microscale gas chromatograph. The realized device enables parts-per-billion detection and quantitative analysis of volatile organic compounds (VOCs) in indoor air. Macroscale systems typically employ a tube preconcentrator and a thermal desorption unit to extract and inject samples. The consequent human intervention to interface the two components precludes macrosystems from performing real-time analysis. Our 44mm3 preconcentrator is the first to take advantage of silicon micromachining to embody and perform exhaustive sample extraction and injection in a single unit. The preconcentrator extracts VOCs using highly adsorptive, granular forms of graphitized carbon black and carbon molecular sieve with surface areas of 100 to 1000 m2/g. The 180 to 212mum-diameter carbons reside in 450mum-deep silicon cavities that have integrated heating and temperature sensing so that injection proceeds by heating the device to 300°C with 1.8W to thermally desorb the VOCs at part-per-million concentrations. The keys to device construction are a new adsorbent-solvent filling technique and a solvent-tolerant wafer-level Si-Au eutectic bonding technology. The product is the first granular adsorbent preconcentrator integrated at the wafer level. This is also the first preconcentrator with proven exhaustive extraction and injection performance: decane at 1 x 10-6 g/L is preconcentratod to 346 x 10-6 g/L and 1.13L of octane at 42.89 x 10-6 g/L and 25 mL/min is extracted before breakthrough. With 3:1 split-flow, 8 mL/min, and 30s stop-flow heating to 300°C, 100 ms injections are demonstrated, representing an order of magnitude reduction in peak width from predecessor prototypes. A mass transfer model, the first for any microscale preconcentrator, is developed to describe both adsorption and desorption behaviors. The physically intuitive model uses implicit and explicit finite differences to numerically solve the partial differential equations. The model is applied to variable-concentration decane adsorption and desorption at 75°C and 25 mL/min to extract Langmuir adsorption isotherm parameters from effluent curve measurements where properties are unknown a priori.Ph.D.Analytical chemistryApplied SciencesElectrical engineeringPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126115/2/3237924.pd