A zone-heated gas chromatographic microcolumn: Energy efficiency.

A microfabricated separation column designed for ultimate use in a wearable gas chromatographic micro-analytical system (μGC) for analyzing mixtures of airborne volatile organic compounds (VOC) is described. The monolithic μcolumn chip measures 7.1 × 2.7 × 0.075 cm and contains a 6-m long, 250 × 140 μm deep-reactive-ion-etched Si channel with a Pyrex cap, wall-coated with a polydimethylsiloxane (PDMS) stationary phase. Along the channel are three serial 2-m long spiral segments, each with an independent integrated resistive heater and thermal isolation features etched in the substrate. By turning the segment heaters on and off at strategic points during a separation, significant energy savings could be realized relative to heating the entire chip simultaneously (i.e., globally), with no loss in chromatographic resolution. A classical lumped element model was used as the basis for simulations of energy consumption, and a published band trajectory model was used to estimate analyte residence times in each segment. Four simple mixtures of volatile organic chemicals were used to evaluate the models and assess the energy consumed for zone heating and global heating under isothermal and temperature-ramped conditions. Modeled reductions in the required energy per analysis using zone (vs. global) heating ranged from 14 to 31% among the cases considered, depending on the heating profile (i.e., isothermal or ramped), heating schedule, and the retention times of the analytes in the mixture. Modeled energy reductions tended to underestimate experimental reductions, but differed by <2% in all cases considered. This approach to μcolumn design and operation shows promise for extending battery life in wearable μGC instrumentation. [ABSTRACT FROM AUTHOR]