A 3.0-m-long, 150-microm-wide, 240-microm-deep channel etched in a 3.2-cm-square silicon chip, covered with a Pyrex wafer, and coated with a dimethyl polysiloxane stationary phase is used for the GC separation of volatile organic compounds. The column, which generates approximately 5500 theoretical plates, is temperature-programmed in a conventional convection oven. The column is connected through a heated transfer line to a microfabricated differential mobility spectrometer. The spectrometer incorporates a 63Ni source for atmospheric-pressure chemical ionization of the analytes. Nitrogen or air transport gas (flow 300 cm(3)/min) drives the analyte ions through the cell. The spectrometer operates with an asymmetric radio frequency (RF) electric field between a pair of electrodes in the detector cell. During each radio frequency cycle, the ion mobility alternates between a high-field and a low-field value (differential mobility). Ions oscillate between the electrodes, and only ions with an appropriate differential mobility reach a pair of biased collectors at the downstream end of the cell. A compensation voltage applied to one of the RF electrodes is scanned to allow ions with different differential mobilities to pass through the cell without being annihilated at the RF electrodes. A unique feature of the device is that both positive and negative ions are detected from a single experiment. The combined microfabricated column and detector is evaluated for the analysis of volatile organic compounds with a variety of functionalities.

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The design and performance evaluation of a two-stage resistively-heated thermal modulator that does not use any cryogenic consumables is described. A commercially available piece of stainless-steel wall coated capillary column is used as the modulator. Cooling of the modulator is provided by a two-stage refrigeration unit with an external heat exchanger and a closed-loop recirculating air system. The modulator is resistively heated by sending a current pulse through each stage of the modulator tube. Studies evaluating the modulator trapping efficiency, cooling efficiency, desorption efficiency, and resultant peak shape are presented and discussed. Performance evaluation of this modulator is compared to similar single-stage modulators, both qualitatively and quantitatively, and has exceeded the previous data reported for the single stage versions. GC Â GC chromatograms of a C 6 -C 20 n-alkane mixture, a petroleum sample, and a 38-component mixture of aliphatic, aromatic, and halogenated volatile organic compounds are provided. This two-stage modulator proves to be a viable alternative to commercially available modulators which require large amounts of cryogenic fluids.

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A microfabricated ultrafast GC×GC system, coupled with highly sensitive, low-power NEMS resonators, with handheld form factor for civilian, military, and space applications.