Gas chromatography is widely used to identify and quantify volatile organic compounds for applications ranging from environmental monitoring to homeland security. We investigate a new architecture for microfabricated gas chromatography systems that can significantly improve the range, speed, and efficiency of such systems. By using a cellular approach, it performs a partial separation of analytes even as the sampling is being performed. The subsequent separation step is then rapidly performed within each cell. The cells, each of which contains a preconcentrator and separation column, are arranged in progression of retentiveness. While accommodating a wide range of analytes, this progressive cellular architecture (PCA) also provides a pathway to improving energy efficiency and lifetime by reducing the need for heating the separation columns. As a proof of concept, a three-cell subsystem (PCA3mv) has been built; it incorporates a number of microfabricated components, including preconcentrators, separation columns, valves, connectors, and a carrier gas filter. The preconcentrator and separation column of each cell are monolithically implemented as a single chip that has a footprint of 1.8 × 5.2 cm2. This subsystem also incorporates two manifold arrays of microfabricated valves, each of which has a footprint of 1.3 × 1.4 cm2. Operated together with a commercial flame ionization detector, the subsystem has been tested against polar and nonpolar analytes (including alkanes, alcohols, aromatics, and phosphonate esters) over a molecular weight range of 32–212 g/mol and a vapor pressure range of 0.005–231 mmHg. The separations require an average column temperature of 63–68 °C within a duration of 12 min, and provide separation resolutions >2 for any two homologues that differ by one methyl group.
We present a new thermomechanical method and a platform to measure the phase transition temperature at microscale. A thin film metal sensor on a membrane simultaneously measures both temperature and mechanical strain of the sample during heating and cooling cycles. This thermomechanical principle of operation is described in detail. Physical hydrogel samples are prepared as a disc-shaped gels (200 μm thick and 1 mm diameter) and placed between an on-chip heater and sensor devices. The sol-gel transition temperature of gelatin solution at various concentrations, used as a model physical hydrogel, shows less than 3% deviation from in-depth rheological results. The developed thermomechanical methodology is promising for precise characterization of phase transition temperature of thermogels at microscale.
This paper reports the design, microfabrication, and experimental evaluation of a monolithic Si-micromachined four-stage Knudsen pump (KP) suitable for microscale gas chromatography (µGC) applications. Without moving parts, KPs generate gas flow by leveraging free molecular flow against a temperature gradient in narrow channels. In this work the KP consists of four monolithically integrated stages that are fabricated by a five-mask lithographic process; each pump stage is micromachined into a silicon-on-insulator wafer and fluidically connected to adjacent stages by channels etched into glass wafers that are bonded above and below the silicon wafer. The pumping channels are densely arrayed, vertically oriented, 1.2 µm-wide rectangular channels with 10 nm thick Al2O3 sidewalls. The four-stage KP has a footprint of 5 × 7.5 mm2. While operating at ambient atmospheric pressure, the pump provides a blocking pressure of ≈3.3 kPa and a maximum air flow rate of ≈0.75 sccm with 1.2 W input power. (The experimental results match the modeling with <30% discrepancy.) Such performance is suitable for providing gas flow in µGCs.
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