Designing Thermal Modulators for Portable GC x GC Systems.

Abstract

Microelectromechanical systems (MEMS) have the advantage of scale and can be manufactured in bulk. One of the active areas of MEMS research is the development of micro-scale comprehensive two-dimensional gas chromatography (μGC×μGC).

Our previous work demonstrated the development of the first microscale thermal modulator(µTM) for use in GC×GC. However, our demonstration was limited to very simple mixtures. Rapid, GC×GC separations by use of a mid-point µTM are demonstrated and the effects of various µTM design and operating parameters on performance are characterized here. A 9 compound structured chromatogram and a 21-component separation was achieved in < 3 min.

Next we demonstrate GC × GC with all microfabricated components. The first dimension consists of two series coupled μcolumn chips with etched channels, with a PDMS stationary phase. The second dimension consists of a μcolumn chip with either a trigonal tricationic room-temperature ionic liquid (RTIL) or a commercial poly(trifluoropropylmethyl siloxane) (OV-215) stationary phase. Conventional injection methods and flame ionization detection were used.

Current conventional thermal modulators can achieve FWHH of modulated peaks of ~ 10 ms, which is necessary to obtain optimum peak capacity in GC×GC by using cryogenic consumables or high amounts of power. However, since we are limited in the amount of cooling power we can use, we need to understand the fundamental physics governing the thermal modulation, and optimize our modulators. Hence we developed a theoretical model of single-stage TM with the aim to elucidate factors leading to improvements in GC×GC analyses. Model predictions were compared with experimental data obtained using our μTM operating as a single-stage TM and excellent match is obtained.

To make a more realistic model, we demonstrated the physics behind the operation of a two-stage modulator. We show that parameters such as the time constant of modulation can be used to reduce the FWHH, breakthrough and hence improve the peak capacity of the GC×GC significantly. Going forward, this theory can be used to optimize the performance of the thermal modulator and coupled with thermal simulations to design the next generation of thermal modulators.