Gustavo Serrano

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.

Comprehensive two-dimensional gas chromatographic separations with a microfabricated thermal modulator

Rapid, comprehensive two-dimensional gas chromatographic (GC × GC) separations by use of a microfabricated midpoint thermal modulator (μTM) are demonstrated, and the effects of various μTM design and operating parameters on performance are characterized. The two-stage μTM chip consists of two interconnected spiral etched-Si microchannels (4.2 and 2.8 cm long) with a cross section of 250 × 140 μm(2), an anodically bonded Pyrex cap, and a cross-linked wall coating of poly(dimethylsiloxane) (PDMS). Integrated heaters provide rapid, sequential heating of each μTM stage, while a proximate, underlying thermoelectric cooler provides continual cooling. The first-dimension column used for GC × GC separations was a 6 m long, 250 μm i.d. capillary with a PDMS stationary phase, and the second-dimension column was a 0.5 m long, 100 μm i.d. capillary with a poly(ethylene glycol) phase. Using sets of five to seven volatile test compounds (boiling point ≤174 °C), the effects of the minimum (T(min)) and maximum (T(max)) modulation temperature, stage heating lag/offset (O(s)), modulation period (P(M)), and volumetric flow rate (F) on the quality of the separations were evaluated with respect to several performance metrics. Best results were obtained with a T(min) = -20 °C, T(max) = 210 °C, O(s) = 600 ms, P(M) = 6 s, and F = 0.9 mL/min. Replicate modulated peak areas and retention times were reproducible to <5%. A structured nine-component GC × GC chromatogram was produced, and a 21 component separation was achieved in <3 min. The potential for creating portable μGC × μGC systems is discussed.

Evaluation of a microfabricated thermal modulator for comprehensive two-dimensional microscale gas chromatography.

A microfabricated thermal modulator (μTM) designed for ultimate use in a comprehensive two-dimensional microscale gas chromatography (μGC × μGC) system is evaluated. The 2-stage device measures 13 mm (l) × 6 mm (w) × 0.5 mm (h) and consists of two interconnected serpentine etched-Si microchannels suspended from a thin Pyrex cap and wall-coated with PDMS (polydimethylsiloxane). The chip is mounted within a few tens of micrometers of a thermoelectric cooler that maintains both stages at a baseline temperature between -35 and -20 °C in order to focus analytes eluting from an upstream separation column. Each stage is heated to 210 °C sequentially at a rate as high as 2400 °C/s by independent thin-film resistors to inject the analytes in consecutive fractions to a downstream column, and then cooled at a rate as high as -168 °C/s. The average power dissipation is only ∼10 W for heating and 21 W for cooling without using consumable materials. In this study, the outlet of the μTM is connected directly to a flame ionization detector to assess its performance. Following a demonstration of basic operation, the modulated peak amplitude enhancement (PAE) and full-width-at-half-maximum (fwhm) are evaluated for members of a series of n-alkanes (C(6)-C(10)) as a function of the rim and stage temperatures; modulation period, phase, and offset; analyte concentration; and carrier-gas flow rate. A PAE as high as 50 and a fwhm as narrow as 90 ms are achieved for n-octane under optimized conditions.

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.

Adaptable chip-level microfluidic packaging for a micro-scale gas chromatograph

In this paper, we present a robust and adaptable technique to integrate microfluidics with an on-chip thiolate-monolayer-protected gold nanoparticle coated chemiresistorarray for vapor analyte detection in a micro-scale gas chromatograph (μGC). The process involves mounting a sensing chip and capillary tubes within a silicon “extension carrier” (EC), capping the chemiresistor-array with a glass lid, and sealing the microfluidic package with non-sorbent epoxy. The stability and efficacy of the integrated detector cell is elucidated by consistent chip responses induced by the diffusion of vapor analytes though the detector cell.

Comprehensive two-dimensional gas chromatography using a MEMS thermal modulator

This paper presents the first comprehensive two-dimensional gas chromatographic (GC×GC) separations of volatile organic compound (VOC) mixtures employing a microfabricated mid-point thermal modulator (μTM). The first-dimension column was coated with a non-polar polydimethylsiloxane (PDMS) film and the second-dimension column was coated with the polar polyethylene glycol (PEG) film. μTM heating and cooling rates of 2400 ° C/s and -168 °C/s, respectively, were achieved. The effects of system flow rate and the maximum and minimum μTM temperatures on the resolution and signal enhancement are explored with small sets of 5-7 VOCs for a fixed modulation period of 6 s. A structured 2-D chromatogram is produced and a 15-VOC GC×GC separation is achieved in 185 sec. This low-power, consumable-free μTM provides performance rivaling many commercial macroscale thermal modulators.

CMOS monolithic chemiresistor array with microfluidic channel for micro gas chromatograph

A monolithic chemiresistor (CR) array micro-system with microfluidic channel for micro gas chromatograph (μGC) is presented in this paper. A CMOS readout chip was designed for amplifying and conditioning the signal of a 4×2 MPN-coated CR array fabricated on the surface of the CMOS chip. A micro glass lid with input and output capillary tubes was developed as a gas flow channel and mounted on the CMOS array, providing an interface to a micro flow column in a μGC platform. After all of the CMOS-compatible processing, the monolithic CR array was tested within a GC platform, and both the CR array and circuit were demonstrated to function as designed.