μGC × μGC: Comprehensive Two-Dimensional Gas Chromatographic Separations with Microfabricated Components

Abstract

The development and characterization of a microanalytical subsystem comprising silicon-micromachined first- and second-dimension separation columns and a silicon-micromachined thermal modulator (μTM) for comprehensive two-dimensional (i.e., μGC × μGC) separations are described. The first dimension consists of two series-coupled 3.1 cm × 3.1 cm μcolumn chips with etched channels 3 m long and 250 μm × 140 μm in cross section, wall-coated with a poly(dimethylsiloxane) (PDMS) stationary phase. The second dimension consists of a 1.2 cm × 1.2 cm μcolumn chip with an etched channel 0.5 m long and 46 μm × 150 μm in cross section wall-coated with either a trigonal tricationic room-temperature ionic liquid (RTIL) or a commercial poly(trifluoropropylmethyl siloxane) (OV-215) stationary phase. The two-stage, cryogen-free μTM consists of a Si chip containing two series-coupled, square spiral channels 4.2 cm and 2.8 cm long and 250 μm × 140 μm in cross section wall-coated with PDMS. Conventional injection methods and flame ionization detection were used. Temperature-ramped separations of a simple alkane mixture using the RTIL-coated second-dimension ((2)D) μcolumn produced reasonably good peak shapes and modulation numbers; however, strong retention of polar compounds on the RTIL-coated (2)D μcolumn led to excessively broad peaks with low (2)D resolution. Substituting OV-215 as the (2)D μcolumn stationary phase markedly improved the performance, and a structured 22 min chromatogram of a 36-component mixture spanning a vapor pressure range of 0.027 to 13 kPa was generated with modulated peak fwhm (full width at half-maximum) values ranging from 90 to 643 ms and modulation numbers of 1-6.

Comprehensive two-dimensional gas chromatographic separations with a temperature programmed microfabricated thermal modulator

“…Inspired by this approach, we were interested in incorporating a similar feature into the microfabricated thermal modulator (μTM) on which we have reported recently [29][30][31][32][33]. First described in 2010 [29,30], this µTM is cryogen-free and requires much less power to operate than conventional TMs.…”

“…It consists of a single, Pyrex-sealed Si microchannel with two thermally isolated spiral sections, or stages, each with independent thin-metal-film heaters. The µTM is mounted on a solid-state thermoelectric cooler (TEC) capable of maintaining T min as low as -35 o C, and it can be heated rapidly to > 250 o C and then cooled again with modulation periods, P m , as short as 5 s. We have used this device in GC  GC separations with conventional capillary columns [30,31], and in GC  GC separations with microfabricated columns [32] and, most recently, with a polymer-coated, microfabricated optofluidic ring resonator (µOFRR) as the detector [33].…”

“…Recognizing the constraint on the analyte volatility range over which effective µGC  µGC separations could be performed with fixed values of T min and T max , we first explored a passive approach to ramping these TM temperatures by placing the device inside the GC oven during a temperature programmed separation [32]. As the oven temperature increased, T min increased because of the reduction in heat dissipation from the TEC heat sink and T max also increased because of the improved efficiency of heating the stages at constant applied voltage.…”

“…Synthesis is relatively straightforward and although the surface pretreatment is tricky it does not involve high temperatures [32,37]. RTILs of this type have been used in the 2 D column for GC × GC separations [32,39], and they were, therefore, pursued in this study as phases for our µTM.…”

“…RTILs of this type have been used in the 2 D column for GC × GC separations [32,39], and they were, therefore, pursued in this study as phases for our µTM.…”