Metal-Organic Framework Stationary Phases for One- and Two-Dimensional Micro-Gas Chromatographic Separations of Light Alkanes and Polar Toxic Industrial Chemicals.

Despite promising advances with metal-organic frameworks (MOFs) as stationary phases for chromatography, the application of MOFs for one- and two-dimensional micro-gas chromatography (µGC and µGC × µGC) applications has yet to be shown. We demonstrate for the first time, µGC columns coated with two different MOFs, HKUST-1 and ZIF-8, for the rapid separation of high volatility light alkane hydrocarbons (natural gas) and determined the partition coefficients for toxic industrial chemicals, using µGC and µGC × µGC systems. Complete separation of natural gas components, methane through pentane, was completed within 1 min, with sufficient resolution to discriminate n-butane from i-butane. Layer-by-layer controlled deposition cycles of the MOFs were accomplished to establish the optimal film thickness, which was validated using GC (sorption thermodynamics), quartz-crystal microbalance gravimetric analysis and scanning electron microscopy. Complete surface coverage was not observed until after ~17 deposition cycles. Propane retention factors with HKUST-1-coated µGC and a state-of-the-art polar, porous-layer open-tubular (PLOT) stationary phase were approximately the same at ~7.5. However, with polar methanol, retention factors with these two stationary phases were 748 and 59, respectively, yielding methanol-to-propane selectivity factors of ~100 and ~8, respectively, a 13-fold increase in polarity with HKUST-1. These studies advance the applications of MOFs as µGC stationary phase.

Thermodynamic Studies on a Hydrogen Bonded Acidic 3,5-Bis(trifluoromethyl)phenol-functionalized Polymer as a Gas Chromatography Stationary Phase for Selectively Speciating Chemical Warfare Agents.

We describe for the first time hydrogen bonded acid (HBA) polymer, poly{methyl[3-(2-hydroxyl, 4,6-bistrifluoromethyl)phenyl]propylsiloxane}, (DKAP), as stationary phase for gas chromatography (µGC) of organophosphate (OP), chemical warfare agent (CWA) surrogates, dimethylmethylphosphonate (DMMP), diisopropylmethylphosphonate (DIMP), diethylmethylphosphonate (DEMP), and trimethylphosphate (TMP), with high selectivity. Absorption of OPs to DKAP was one-to-several orders of magnitude higher relative to commercial polar, mid-polar, and nonpolar stationary phases. We also present for the first-time thermodynamic studies on the absorption of OP vapors and quantitative binding energy data for interactions with various stationary phases. These data help to identify the best pair of hetero-polar columns for a two-dimensional GC system, employing a nonpolar stationary phase as GC1 and DKAP as the GC2 stationary phase, for selective and rapid field detection of CWAs.

Analyte discrimination from chemiresistor response kinetics.

Chemiresistors are polymer-based sensors that transduce the sorption of a volatile organic compound into a resistance change. Like other polymer-based gas sensors that function through sorption, chemiresistors can be selective for analytes on the basis of the affinity of the analyte for the polymer. However, a single sensor cannot, in and of itself, discriminate between analytes, since a small concentration of an analyte that has a high affinity for the polymer might give the same response as a high concentration of another analyte with a low affinity. In this paper we use a field-structured chemiresistor to demonstrate that its response kinetics can be used to discriminate between analytes, even between those that have identical chemical affinities for the polymer phase of the sensor. The response kinetics is shown to be independent of the analyte concentration, and thus the magnitude of the sensor response, but is found to vary inversely with the analyte's saturation vapor pressure. Saturation vapor pressures often vary greatly from analyte to analyte, so analysis of the response kinetics offers a powerful method for obtaining analyte discrimination from a single sensor.

Master transduction curve for field-structured chemiresistor calibration.

Chemiresistors are gas sensors for volatile organic compounds that are composed of conducting particle networks in a polymer matrix. In the presence of an analyte that is compatible with the polymer phase, the sensor conductance decreases as the analyte is absorbed, eventually reaching a steady-state value that is a measure of the analyte's concentration. The response curve, which is the relationship between steady-state conductance and analyte activity (normalized concentration), is strongly dependent on both the chemical affinity of the analyte for the polymer and the stress field within the chemiresistor composite. Calibration of an individual sensor would seem to necessitate mapping out the response curve for each analyte of interest, a tedious and expensive proposition. In a recent paper, we have shown that the transduction curve of any particular sensor is a function of polymer swelling alone, regardless of the chemical nature of the analyte. This master transduction curve implies that sensor calibration requires only a knowledge of the polymer mass-sorption isotherm for any set of analytes of interest, data that can be collected once and for all. Any single analyte can then be used to calibrate the response of a particular sensor as a function of analyte activity, and the response to other analytes can be predicted. As a corollary, a calibrated sensor can be used to determine the mass-sorption data for any other analyte of interest. In this paper, we provide a detailed description of the construction of the master transduction curve, show how this curve can be used to measure polymer sorption with a calibrated chemiresistor, and demonstrate the use of a single analyte to calibrate sensors of disparate sensitivities and predict their response to two other analytes.

Strain-tunable chemiresistor.

We have developed a resistance-based chemical sensor (chemiresistor) for volatile organic compounds whose sensitivity can be reversibly increased over a range of nearly 2 decades by the application of tensile strain. This polymer-based sensor is comprised of Au-plated magnetic particles structured into conducting chains by the application of a magnetic field during the curing of the prepolymer resin. The resistance of this field-structured composite increases when an analyte vapor swells the polymer and reduces the contact pressure between particles. We have found that applying a tensile strain increases both the sensor resistance and sensitivity, as defined by its relative resistance change. This increase in sensitivity is a smooth, continuous function of the applied strain, and the effect is fully reversible. Sensitivity tuning enables the response curve of the sensor to be dynamically optimized for sensing analytes over a wide concentration range.