Comprehensive Two-Dimensional Gas Chromatographic Separations with a Microfabricated Thermal Modulator
Rapid, comprehensive two-dimensional gas chromatographic (GC ? GC) separations by use of a microfabricated midpoint thermal modulator (?TM) are demonstrated, and the effects of various ?TM design and operating parameters on performance are characterized. The two-stage ?TM chip consists of two interconnected spiral etched-Si microchannels (4.2 and 2.8 cm long) with a cross section of 250 ? 140 ?m2, an anodically bonded Pyrex cap, and a cross-linked wall coating of poly(dimethylsiloxane) (PDMS). Integrated heaters provide rapid, sequential heating of each ?TM stage, while a proximate, underlying thermoelectric cooler provides continual cooling. The first-dimension column used for GC ? GC separations was a 6 m long, 250 ?m i.d. capillary with a PDMS stationary phase, and the second-dimension column was a 0.5 m long, 100 ?m i.d. capillary with a poly(ethylene glycol) phase. Using sets of five to seven volatile test compounds (boiling point ≤174 °C), the effects of the minimum (Tmin) and maximum (Tmax) modulation temperature, stage heating lag/offset (Os), modulation period (PM), and volumetric flow rate (F) on the quality of the separations were evaluated with respect to several performance metrics. Best results were obtained with a Tmin = ?20 °C, Tmax = 210 °C, Os = 600 ms, PM = 6 s, and F = 0.9 mL/min. Replicate modulated peak areas and retention times were reproducible to <5%. A structured nine-component GC ? GC chromatogram was produced, and a 21 component separation was achieved in <3 min. The potential for creating portable ?GC ? ?GC systems is discussed.