Polymer-coated micro-optofluidic ring resonator detector for a comprehensive two-dimensional gas chromatographic microsystem: μGC × μGC–μOFRR

Abstract

We describe first results from a micro-analytical subsystem that integrates a detector comprising a polymer-coated micro-optofluidic ring resonator (μOFRR) chip with a microfabricated separation module capable of performing thermally modulated comprehensive two-dimensional gas chromatographic separations (μGC ×μGC) of volatile organic compound (VOC) mixtures. The 2 × 2 cm μOFRR chip consists of a hollow, contoured SiO(x) cylinder (250 μm i.d.; 1.2 μm wall thickness) grown from a Si substrate, and integrated optical and fluidic interconnection features. By coupling to a 1550 nm tunable laser and photodetector via an optical fiber taper, whispering gallery mode (WGM) resonances were generated within the μOFRR wall, and shifts in the WGM wavelength caused by transient sorption of eluting vapors into the PDMS film lining the μOFRR cylinder were monitored. Isothermal separations of a simple alkane mixture using a PDMS coated 1st-dimension ((1)D) μcolumn and an OV-215-coated 2nd-dimension ((2)D) μcolumn confirmed that efficient μGC ×μGC-μOFRR analyses could be performed and that responses were dominated by film-swelling. Subsequent tests with more diverse VOC mixtures demonstrated that the modulated peak width and the VOC sensitivity were inversely proportional to the vapor pressure of the analyte. Modulated peaks as narrow as 120 ms and limits of detection in the low-ng range were achieved. Structured contour plots generated with the μOFRR and a reference FID were comparable.

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

“…In a departure from our previous μTM mounting schemes [29][30][31][32][33], a small swatch of thermally conductive silicone material (Sil-Pad, Henkel, Chanhassen, MN) was placed against the heater on the Pyrex side of each stage, and held there with a thin film of thermal paste. Two small, square slabs of Si were then coated on both sides with thermal paste and placed on top of the Sil-Pad swatches.…”

“…The two-stage µTM has been described previously [29][30][31][32][33]. The μTM used for most testing was statically coated with PDMS from a solution that also contained 1% (w/w) dicumyl peroxide as the crosslinking agent using a published procedure [40,41].…”

“…An unavoidable consequence of the method is that the connecting capillaries are coated with un-crosslinked PDMS. The RTIL μTM was also statically coated, following a NaCl surface pretreatment [33]. The RTIL concentration was controlled to yield an average nominal wall-coating thickness of 0.07 μm.…”

“…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].…”