Comprehensive multidimensional gas chromatography and modulator development for portable instrumentation.

Abstract

The goal of this research was to develop technology that will facilitate high-speed, multidimensional, low-resource, and portable gas chromatography (GC) instrumentation. Current commercial GCxGC instruments are very powerful but are large and require expensive cryogenic consumables. Developing a portable GCxGC will reduce the instrument footprint and consumable requirements in addition to providing onsite analysis capabilities to expand the potential application range for GCxGC analysis. The combined separation power of a microfabricated separation column and microfabricated differential mobility spectrometer (DMS) is demonstrated. The additional separation mechanism inherent in the DMS provides increased peak capacity and produces further information for chemical identification. The microfabricated components demonstrate the separation of 45 components in 400 s with vast decreases in system footprint compared to conventional systems. A model was developed for the prediction of band elution times for GCxGC systems consisting of a primary column, secondary column, and modulator with independent inner diameters. Comparisons between modeled and experimental data are provided for alkane species which show excellent model prediction of retention time for low-molecular weight compounds, while increasingly underestimating the retention time for heavier compounds. A low-resource thermal modulator requiring no cryogenic liquids is evaluated for its performance in regards to the flow rate through the modulator and modulation period. The modulator was used in a GCxGC system to successfully analyze headspace samples of U.S. currency by detecting a select group of ink decomposition marker compounds. Attempts to create high-speed modulators capable of producing sub-20 ms peak width plugs of analyte for very fast (∼100 ms) second column separations to achieve truly high-speed GCxGC separations are described. Two styles of high-speed modulators are presented; a narrow-bore, uncoated steel tube modulator and a microfabricated channel modulator. Both employed cold-jet cooling with resistive heating to provide narrow band slices (17 ms peak width achieved with steel tube modulator). Valve modulators capable of sub-300 ms modulation periods for high-speed GCxGC separations were investigated using Sandia-proprietary valves. Experiments demonstrating the modulation of methane, peak area conservation, and an 18-component separation completed in 9 s are described. Additional analyses regarding modulated-band peak-width and overall peak capacity are included.