Hungwei Chang

Microfabricated Gas Chromatograph for the Selective Determination of Trichloroethylene Vapor at Sub-Parts-Per-Billion Concentrations in Complex Mixtures

A complete field-deployable microfabricated gas chromatograph (μGC) is described, and its adaptation to the analysis of low- and subparts-per-billion (ppb) concentrations of trichloroethylene (TCE) vapors in complex mixtures is demonstrated through laboratory testing. The specific application being addressed concerns the problem of indoor air contamination by TCE vapor intrusion. The μGC prototype employs a microfabricated focuser, dual microfabricated separation columns, and a microsensor array. These are interfaced to a nonmicrofabricated front-end pretrap and high-volume sampler module to reduce analysis time and limits of detection (LOD). Selective preconcentration and focusing are coupled with rapid chromatographic separation and multisensor detection for the determination of TCE in the presence of up to 45 interferences. Autonomous operation is possible via a laptop computer. Preconcentration factors as high as 500 000 are achieved. Sensitivities are constant over the range of captured TCE masses tested (i.e., 9-390 ng), and TCE is measured in a test atmosphere at 120 parts-per-trillion (ppt), with a projected LOD of 40 ppt (4.2 ng captured, 20 L sample) and a maximum sampling + analytical cycle time of 36 min. Short- and medium-term (1 month) variations in retention time, absolute responses, and response patterns are within acceptable limits.

Microfabricated Gas Chromatograph for Rapid, Trace-Level Determinations of Gas-Phase Explosive Marker Compounds

A prototype microfabricated gas chromatograph (μGC) adapted specifically for the rapid determination of selected gas-phase marker compounds of the explosive 2,4,6-trinitrotoluene (TNT) at sub-parts-per-billion (<ppb) concentrations in complex mixtures is described. Si-microfabricated focuser, separation column, and sensor array components are integrated with a high-volume sampler of conventional construction to reduce analysis time and the limit of detection (LOD). The primary markers selected as target analytes were 2,4-dinitrotoluene (2,4-DNT, a persistent impurity of TNT) and 2,3-dimethyl-2,3-dinitrobutane (DMNB, a taggant), along with 2,6-dinitrotoluene (2,6-DNT, a less-prominent TNT impurity), which was also included in numerous tests. Selective preconcentration, on-column focusing, temperature-programmed chromatographic separation, and sensor array detection/recognition facilitated determinations of the primary markers in the presence of 20 (or more) interferences within ∼2 min under laptop control. Estimated LODs of 2.2, 0.48, and 0.86 ng were achieved for DMNB, 2,6-DNT, and 2,4-DNT, respectively, which correspond to 0.30, 0.067, and 0.12 ppb in each 1-L air sample collected.

Multi-stage preconcentrator/focuser module designed to enable trace level determinations of trichloroethylene in indoor air with a microfabricated gas chromatograph

This article describes the development and characterization of a multi-stage preconcentrator/focuser (PCF) module designed to be integrated with a microfabricated gas chromatograph (µGC) for autonomous, in situ determinations of volatile organic compounds. The PCF module has been optimized specifically for the determination of trichloroethylene (TCE) vapors at low- or sub-parts-per-billion concentrations in the presence of common indoor air co-contaminants in residences at risk of vapor intrusion (VI) from surrounding TCE-contaminated soil. It consists of three adsorbent-packed devices arranged in series: a pre-trap of conventional (tubular metal) design for capturing interferences with vapor pressures <3 torr; a high-volume sampler, also of conventional design, for capturing (and transferring) TCE and other compounds with vapor pressures within the range of ~3 to 95 torr; and a microfocuser (µF) consisting of a micromachined Si chamber with an integrated microheater for focusing and injecting samples into the separation module. The adsorbent masses, sampling and desorption flow rates, and heating profiles required for selective, quantitative capture and transfer/injection of TCE are determined for each of the devices, and the assembled PCF module is used to analyze a test atmosphere containing 200 parts-per-trillion of TCE and 27 relevant co-contaminants with a conventional downstream capillary column and electron-capture detector. An average TCE transfer efficiency of 107% is achieved for a 20 L air sample, with a preconcentration factor of ~800,000.

A micro gas chromatograph for high-speed determinations of explosive markers

A complete gas chromatographic (μGC) microanalytical system for near-real-time determinations of marker compounds of explosives at low concentrations is demonstrated. This μGC, dubbed INTREPID, uses an adsorbent-packed micromachined DRIE-Si/glass microfocuser, a wall-coated 1 × 1 cm DRIE-Si/glass microcolumn (1-m channel length), and an integrated array of 4 chemiresistors coated with functionalized thiolate-monolayer-protected gold nanoparticle (MPN) interface layers. A high-volume sampler of conventional design is interfaced to the font-end of the microsystem to reduce limits of detection (LOD) and the analytical duty cycle. Instrument control and data acquisition are executed automatically via Labview code run from a laptop computer. The marker compounds consisted of the TNT byproducts, 2,4- and 2,6-dinitrotoluene, and the taggant, 2,3-dimethyl- 2,3-dinitrobutane. n-Tridecane was used as a representative interference. The combination of retention time and response pattern facilitated the identification of each marker. A full analysis was achieved in &#60; 3.5 min with LODs in the low- or sub- parts-per-billion range for a preconcentrated air-sample volume of 1 L.

Microfabricated Gas Chromatograph for On-Site Determination of Trichloroethylene in Indoor Air Arising from Vapor Intrusion. 1. Field Evaluation

Results are presented of inaugural field tests of two identical prototype microfabricated gas chromatographs (μGC) adapted for the in situ determination of trichloroethylene (TCE) in indoor air in support of vapor intrusion (VI) investigations. Each μGC prototype has a pretrap and partially selective high-volume sampler of conventional design, a micromachined-Si focuser for injection, dual micromachined-Si columns for separation, and an integrated array of four microscale chemiresistors with functionalized gold nanoparticle interface films for multichannel detection. Scrubbed ambient air is used as the carrier gas. Field-generated calibration curves were linear for injected TCE masses of 26-414 ng (4.8-77 ppb·L; r(2) > 0.98) and the projected single-sensor detection limit was 0.052 ppb for an 8-L air sample collected and analyzed in 20 min. Consistent performance between the prototypes and good medium-term stability were shown. Above the mitigation action level (MAL) of 2.3 ppb for the field-test site, μGC TCE determinations fell within ±25% of those from the reference method for 21 of 26 measurements, in the presence of up to 37 documented background VOCs. Below the MAL, positive biases were consistently observed, which are attributable to background VOCs that were unresolvable chromatographically or by analysis of the sensor-array response patterns. Results demonstrate that this type of μGC instrument could serve the need for routine TCE determinations in VI-related assessment and mitigation efforts.

Prototype micro gas chromatograph for breath biomarkers of respiratory disease

Progress toward a prototype MEMS gas chromatograph (μGC) designed for analyzing complex mixtures of volatile organic compounds (VOC) in breath is described. This μGC, named SPIRON, integrates a multi-stage preconcentrator/injector, a dual-microcolumn separation module, and a chemiresistor array detector with commercial minivalves and a minipump. Here, we describe results showing thermal control of all key system components and then present the rapid separation of two complex mixtures: one containing lung cancer biomarkers and the other containing tuberculosis biomarkers, both in backgrounds of 30 common breath VOCs. The fully assembled SPIRON μGC prototype is also presented.

Field testing of a rugged MEMS gas chromatograph prototype: Selective analysis of trace-level TCE vapors in contaminated homes

Two fully integrated and automatically controlled MEMS gas chromatographs (µGC) were successfully deployed in the field to monitor trichloroethylene (TCE) at trace-level concentrations (0.6–80 ppb) in homes suffering from vapor intrusion (VI) from surrounding TCE-contaminated soil. Each instrument combines a high-volume sampling module (non-microfabricated) with a microanalytical module consisting of a microfocuser, dual microcolumns, and a chemiresistor array detector. A complete sampling and analytical cycle requires 15–30 min, depending on required sensitivity. Limits of detection as low as 0.02 ppb were achieved. TCE was separated from 45 co-contaminants. Use of the prototypes in fixed-site (temporal TCE fluctuations) and portable operating modes (spatial mapping of TCE) was successfully demonstrated. Good agreement with concentrations determined with standard reference methods was achieved.