Microfabricated Passive Preconcentrator/Injector designed for Microscale Gas Chromatography.

Abstract

Microelectromechanical system (MEMS) technology enables the realization of miniaturized lab-on-a-chip analytical systems. One of the active MEMS research areas is microscale chromatography (micro-GC) capable of field-deployable in situ gas analysis. An analytical micro-GC system promises to meet the need for hand-held, low-cost, and low-power instruments analyzing complex organic compounds. Prominent applications of micro-GC include environmental air monitoring, homeland security, military surveillance, and biomedical diagnostics. In particular, a complete micro-GC system comprises at least the following three essential microscale components: (1) a preconcentrator, (2) a column, and (3) a detector. A micromachined preconcentrator plays a major role in the quantitative detection of volatile organic compounds (VOC) in moderately complex mixtures using a micro-GC system. The preconcentrator installed at the front end of an analytical system significantly enhances the detection performance of a micro-GC system by trapping and concentrating analytes. Preconcentrated vapors are thermally desorbed by a heat pulse generated at the heater integrated in the preconcentrator and delivered as a concentrated plug into the downstream separation column by pumping. A narrow injection time-width pulse of analytes by rapid thermal desorption leads to a significant improvement in the separation ability of the system. Thus, there is a strong need for a power-efficient microfabricated preconcentrator capable of achieving high preconcentration and rapid heating. Here, we present design, fabrication, thermal operation, and characterization of a device named the microfabricated passive preconcentrator/injector (micro-PPI), which captures VOC mixtures from the ambient by diffusion without a pump. The micro-PPI consists of two layer structures; the top layer incorporates vertical square diffusion channels for passive vapor sampling; and the bottom layer contains a membrane cavity structure with pillar structures to retain carbon adsorbent granules inside and an integrated heater on its backside for injecting captured VOCs downstream by thermal desorption. Fluidic and heat transfer models are developed to guide the device design to ensure power-efficient sample transfer during the thermal desorption. Comprehensive tests of the micro-PPI demonstrate its general utility to enhance the detection performance of a micro-GC system.