Integrated Micro Gas Chromatographs with High-Flow Knudsen Pumps
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Environmental gas sensing typically requires both sensitivity and specificity; target vapor species must not only be detected and quantified, but also differentiated from interferents. This mission can be accomplished by micro gas chromatographs (µGCs), which allow preconcentration of samples and subsequent separation of complex vapor mixtures into individual constituents by their specific retention times. This thesis focuses on the system-level design, fabrication, and integration of µGCs, with the ultimate goal of fully microfabricated systems that can be easily manufactured and distributed to end-users. This thesis also explores the optimization of a micro gas pump "“ a critical µGC component, and generally recognized as a challenge for microsystems.
Three generations of integrated Â µGC systems have been designed, fabricated, and evaluated. The iGC1 system demonstrates the feasibility of a low-cost three-mask fabrication approach for a Â µGC including a Knudsen pump, a preconcentrator, a separation column and a microdischarge-based detector, which are integrated in a 4-cm3 stack. The iGC2 system demonstrates a valveless Â µGC architecture, in which a bi-directional Knudsen pump provides reversible gas flow for (multi-stage) preconcentrators, which is essential for quantitative analysis. The iGC3 system replaces the microdischarge-based detectors in iGC1 and iGC2 with complementary capacitive detectors, facilitating a purely electronic interface for the fluidics. Additionally, it is compatible with the use of room air as the carrier gas. The quantitative analysis of 19 chemicals with concentration levels of well below 100 ppb is demonstrated, showing the promise of automated, continuous monitoring of indoor air pollutants.
The pump used in the iGCx systems is a Knudsen pump that uses thermal transpiration and has no moving parts. This thesis describes pumps that use nanoporous media to provide thermal transpiration and generate gas flow for preconcentration and analysis. It also describes an exploratory effort in which lithographically fabricated channels in silicon substrates provide the thermal transpiration. The Si-micromachined Knudsen pumps demonstrate >200 sccm flow rate. To increase the output pressure head, these pumps are arrayed in series, using both a stacked configuration and a planar one. The results show that the pressure and flow characteristics can be tailored over a wide performance range, extending the possible applications beyond Â µGC systems.