Microfabricated thermal modulator for comprehensive two-dimensional micro gas chromatography: design, thermal modeling, and preliminary testing

Abstract

In comprehensive two-dimensional gas chromatography (GC×GC), a modulator is placed at the juncture between two separation columns to focus and re-inject eluting mixture components, thereby enhancing the resolution and the selectivity of analytes. As part of an effort to develop a μGC×μGC prototype, in this report we present the design, fabrication, thermal operation, and initial testing of a two-stage microscale thermal modulator (μTM). The μTM contains two sequential serpentine Pyrex-on-Si microchannels (stages) that cryogenically trap analytes eluting from the first-dimension column and thermally inject them into the second-dimension column in a rapid, programmable manner. For each modulation cycle (typically 5 s for cooling with refrigeration work of 200 J and 100 ms for heating at 10 W), the μTM is kept approximately at −50 °C by a solid-state thermoelectric cooling unit placed within a few tens of micrometres of the device, and heated to 250 °C at 2800 °C s⁻¹ by integrated resistive microheaters and then cooled back to −50 °C at 250 °C s⁻¹. Thermal crosstalk between the two stages is less than 9%. A lumped heat transfer model is used to analyze the device design with respect to the rates of heating and cooling, power dissipation, and inter-stage thermal crosstalk as a function of Pyrex-membrane thickness, air-gap depth, and stage separation distance. Experimental results are in agreement with trends predicted by the model. Preliminary tests using a conventional capillary column interfaced to the μTM demonstrate the capability for enhanced sensitivity and resolution as well as the modulation of a mixture of alkanes.