The fabrication, assembly, and initial testing of a hybrid microfabricated gas chromatograph (microGC) is described. The microGC incorporates capabilities for on-board calibration, sample preconcentration and focused thermal desorption, temperature-programmed separations, and "spectral" detection with an integrated array of microsensors, and is designed for rapid determinations of complex mixtures of environmental contaminants at trace concentrations. Ambient air is used as the carrier gas to avoid the need for on-board gas supplies. The microsystem is plumbed through an etched-Si/glass microfluidic interconnection substrate with fused silica capillaries and employs a miniature commercial pump and valve subsystem for directing sample flow. The latest performance data on each system component are presented followed by first analytical results from the working microsystem. Tradeoffs in system performance as a function of volumetric flow rate are explored. The determination of an 11-vapor mixture of typical indoor air contaminants in less than 90 s is demonstrated with projected detection limits in the low part-per-billion concentration range for a preconcentrated air-sample volume of 0.25 L.

An investigation of the modulation of charge transport through thin films of n-octanethiolate monolayer-protected gold nanoparticles (MPN) induced by the sorption of organic vapors is presented. A model is derived that allows predictions of MPN-coated chemiresistor (CR) responses from vapor-film partition coefficients, and analyte densities and dielectric constants. Calibrations with vapors of 28 compounds collected from an array of CRs and a parallel thickness-shear-mode resonator are used to verify assumptions inherent in the model and to assess its performance. Results afford insights into the nature of the vapor-MPN interactions, including systematic variations in apparent film swelling efficiencies, and show that the model can predict CR responses typically to within 24%. Using CRs of different dimensions, vapor sensitivities are found to be virtually independent of the MPN film volume over a range of 104 (device-area x MPN layer thickness). Sensitivities vary inversely with analyte vapor pressure similarly for the two sensor types, but the CR sensor affords significantly greater signal-to-noise ratios, yielding calculated detection limits in the low-part-per-billion concentration range for several of the analytes tested. The implications of these results for implementing MPN-coated CR arrays as detectors in microanalytical systems are considered.

The laboratory characterization of a novel, second-generation portable gas chromatograph (GC) prototype designed for trace-level determinations of complex mixtures of volatile organic compounds (VOC) is described. The instrument incorporates a small, multi-stage adsorbent preconcentrator/injector (PCI), two series-coupled separation columns with fast, independent temperature-programming capabilities and junction-point pressure/flow control, and a detector consisting of an array of microfabricated chemiresistor (CR) sensors coated with thiolate-monolayer-protected gold nanoparticle films. Response patterns from the CR array are used in conjunction with chromatographic retention times to identify eluting mixture components. Scrubbed ambient air is used as the carrier gas. Enhancements in design relative to a previously reported first-generation prototype instrument have led to significant reductions in limits of detection as well as improvements in resolution, reliability, flexibility, and convenience. Key features of the instrument are characterized, with an emphasis on the tradeoffs in sensor array performance associated with operation at different temperatures and flow rates. The separation of a preconcentrated mixture of 31 VOCs in < 7 minutes is demonstrated. Projected detection limits are in the ppt range for most compounds, assuming a 1 L sample volume.

We describe a belt-mountable prototype instrument containing a gas chromatographic microsystem (μGC) and demonstrate its capability for near-real-time recognition and quantification of volatile organic compounds (VOCs) in moderately complex mixtures at concentrations encountered in industrial workplace environments. The μGC comprises three discrete, Si/Pyrex microfabricated chips: a dual-adsorbent micropreconcentrator–focuser for VOC capture and injection; a wall-coated microcolumn with thin-metal heaters and temperature sensors for temperature-programmed separations; and an array of four microchemiresistors with thiolate-monolayer-protected-Au-nanoparticle interface films for detection and recognition–discrimination. The battery-powered μGC prototype (20 × 15 × 9 cm, ∼2.1 kg sans battery) has on-board microcontrollers and can autonomously analyze the components of a given VOC mixture several times per hour. Calibration curves bracketing the Threshold Limit Value (TLV) of each VOC yielded detection limits of 16–600 parts-per-billion for air samples of 5–10 mL, well below respective TLVs. A 2:1 injection split improved the resolution of early eluting compounds by up to 63%. Responses and response patterns were stable for 5 days. Use of retention-time windows facilitated the chemometric recognition and discrimination of the components of a 21-VOC mixture sampled and analyzed in 3.5 min. Results from a “mock” field test, in which personal exposures to time-varying concentrations of a mixture of five VOCs were measured autonomously, agreed closely with those from a reference GC. Thus, reliable, near-real-time determinations of worker exposures to multiple VOCs with this wearable μGC prototype appear feasible.

Use of electron-beam induced crosslinking (EBIX) to pattern films of thiolate-monolayer-protected gold-nanoparticles (MPNs) on chemiresistor (CR) vapor sensors is described. MPNs with alkyl, cyanoalkyl, phenoxyalkyl, and hydroxyfluoroalkyl thiolate tail groups were patterned on integrated arrays of interdigital electrodes using electron doses of 500-750 C=cm 2 . The dc resis- tances of solvent cast films of these MPNs decrease and the baseline-normalized changes in resistance to each of five organic vapors increase to different degrees with increasing electron-beam dose. Relative responses patterns from an array of MPN-coated CR sensors for the test vapors change after EBIX patterning and the diversity of responses is diminished, on average, but it is still projected to be sufficient for the discrimination of most of the individual test vapors and binary mixtures. Results are rationalized in terms of expected changes in ligand structures and film properties following EBIX patterning using known models of electronic conduction, and vapor-induced changes of conduction, through MPN films. The implications of the results for creating arrays of densely packed MPN-coated CRs as detectors for microanalytical systems are considered.Index Terms-Chemiresistor, electron beam, nanoparticle, sensor array, vapor sensor.