•GC × GC separations were performed with fixed and programmed modulator temperatures.•With fixed Tmin and Tmax, instances of inefficient trapping or re-mobilization occurred.•With programmed Tmin and Tmax breakthrough was minimized, peak widths were <95 ms.•The GC × GC separation of unleaded gasoline was demonstrated.•Replacing PDMS with a trigonal tricationic RTIL in the modulator eliminated bleed.Comprehensive two-dimensional gas chromatography (GC × GC) with a temperature-programmed microfabricated thermal modulator (μTM) is demonstrated. The 0.78 cm2, 2-stage μTM chip with integrated heaters and a PDMS coated microchannel was placed in thermal contact with a solid-state thermoelectric cooler and mounted on top of a bench scale GC. It was fluidically coupled through heated interconnects to an upstream first–dimension (1D) PDMS-coated capillary column and a downstream uncoated capillary or second-dimension (2D) PEG-coated capillary. A mixture of n-alkanes C6–C10 was separated isothermally and the full-width-at-half-maximum (fwhm) values of the modulated peaks were assessed as a function of the computer-controlled minimum and maximum stage temperatures of μTM, Tmin and Tmax, respectively. With Tmin and Tmax fixed at −25 and 100 °C, respectively, modulated peaks of C6 and C7 had fwhm values <53 ms while the modulated peaks of C10 had a fwhm value of 1.3 s, due to inefficient re-mobilization. With Tmin and Tmax fixed at 0 and 210 °C, respectively, the fwhm value for the modulated C10 peaks decreased to 67 ms, but C6 and C7 exhibited massive breakthrough. By programming Tmin from −25 to 0 °C and Tmax from 100 to 220 °C, the C6 and C7 peaks had fwhm values ≤ 50 ms, and the fwhm for C10 peaks remained < 95 ms. Using the latter conditions for the GC × GC separation of a sample of unleaded gasoline yielded resolution similar to that reported with a commercial thermal modulator. Replacing the PDMS phase in the μTM with a trigonal-tricationic room temperature ionic liquid eliminated the bleed observed with the PDMS, but also reduced the capacity for several test compounds. Regardless, the demonstrated capability to independently temperature program this low resource μTM enhances its versatility and its promise for use in bench-scale GC × GC systems.

A vapor sensor comprising a nanoparticle-coated microfabricated optofluidic ring resonator (μOFRR) is introduced. A multilayer film of polyether functionalized, thiolate-monolayer-protected gold nanoparticles (MPN) was solvent cast on the inner wall of the hollow cylindrical SiOx μOFRR resonator structure, and whispering gallery mode (WGM) resonances were generated with a 1550 nm tunable laser via an optical fiber taper. Reversible shifts in the WGM resonant wavelength upon vapor exposure were detected with a photodetector. The μOFRR chip was connected to a pair of upstream etched-Si chips containing PDMS-coated separation μcolumns and calibration curves were generated from the peak-area responses to five volatile organic compounds (VOCs). Calibration curves were linear, and the sensitivities reflected the influence of analyte volatility and analyte-MPN functional group affinity. Sorption-induced changes in film thickness apparently dominate over changes in the refractive index of the film as the determinant of responses for all VOCs. Peaks from the MPN-coated μOFRR were just 20–50% wider than those from a flame ionization detector for similar μcolumn separation conditions, reflecting the rapid response of the sensor for VOCs. The five VOCs were baseline separated in <1.67 min, with detection limits as low as 38 ng.

The development and characterization of a microanalytical subsystem comprising silicon-micromachined first- and second-dimension separation columns and a silicon-micromachined thermal modulator (μTM) for comprehensive two-dimensional (i.e., μGC × μGC) separations are described. The first dimension consists of two series-coupled 3.1 cm × 3.1 cm μcolumn chips with etched channels 3 m long and 250 μm × 140 μm in cross section, wall-coated with a poly(dimethylsiloxane) (PDMS) stationary phase. The second dimension consists of a 1.2 cm × 1.2 cm μcolumn chip with an etched channel 0.5 m long and 46 μm × 150 μm in cross section wall-coated with either a trigonal tricationic room-temperature ionic liquid (RTIL) or a commercial poly(trifluoropropylmethyl siloxane) (OV-215) stationary phase. The two-stage, cryogen-free μTM consists of a Si chip containing two series-coupled, square spiral channels 4.2 cm and 2.8 cm long and 250 μm × 140 μm in cross section wall-coated with PDMS. Conventional injection methods and flame ionization detection were used. Temperature-ramped separations of a simple alkane mixture using the RTIL-coated second-dimension (2D) μcolumn produced reasonably good peak shapes and modulation numbers; however, strong retention of polar compounds on the RTIL-coated 2D μcolumn led to excessively broad peaks with low 2D resolution. Substituting OV-215 as the 2D μcolumn stationary phase markedly improved the performance, and a structured 22 min chromatogram of a 36-component mixture spanning a vapor pressure range of 0.027 to 13 kPa was generated with modulated peak fwhm (full width at half-maximum) values ranging from 90 to 643 ms and modulation numbers of 1–6.

•Comprehensive testing of capacity and desorption/injection was performed with VOC mixtures.•Dual-adsorbent micro-device exhaustively captured VOCs with vapor pressures from ∼0.03 to 13 kPa.•Split-flow injection (2:1) yielded thermally desorbed peaks as narrow as 0.6 s.•A preconcentration factor of >600 was achieved for benzene from a 31 mL air sample.•Selective air sampling, desorption, separation, and analysis of a 14-VOC mixture was demonstrated.This article describes work leading to a microfabricated preconcentrator-focuser (μPCF) designed for integration into a wearable microfabricated gas chromatograph (μGC) for monitoring workplace exposures to volatile organic compounds (VOCs) ranging in vapor pressure from ∼0.03 to 13 kPa at concentrations near their respective Threshold Limit Values. Testing was performed on both single- and dual-cavity, etched-Si μPCF devices with Pyrex caps and integrated resistive heaters, packed with the graphitized carbons Carbopack X (C-X) and/or Carbopack B (C-B). Performance was assessed by measuring the 10% breakthrough volumes and injection bandwidths of a series of VOCs, individually and in mixtures, as a function of the VOC air concentrations, mixture complexity, sampling and desorption flow rates, adsorbent masses, temperature, and the injection split ratio. A dual-cavity device containing 1.4 mg of C-X and 2.0 mg of C-B was capable of selectively and quantitatively capturing a mixture of 14 VOCs at low-ppm concentrations in a few minutes from sample volumes sufficiently large to permit detection at relevant concentrations for workplace applications with the μGC detector that we ultimately plan to use. Thermal desorption at 225 °C for 40 s yielded ≥99% desorption of all analytes, and injected bandwidths as narrow as 0.6 s facilitated efficient separation on a downstream 6-m GC column in <3 min. A preconcentration factor of 620 was achieved for benzene from a sample of just 31 mL. Increasing the mass of C-X to 2.3 mg would be required for exhaustive capture of the more volatile target VOCs at high-ppm concentrations.

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 (