| Summary: | 碩士 === 國立中央大學 === 化學學系 === 104 === Comprehensive two-dimensional gas chromatography (GC×GC) is the preferred choice of analysis for complex volatile organic compounds (VOCs). When coupled with time-of-flight mass (ToF) spectrometry, termed GC×GC-ToF, it becomes a powerful technique to analyze fuels, perfumes, aromas, environmental samples, etc. with complex chemical compositions. Compared to conventional one-dimensional GC, GC×GC greatly enhances peak capacity via the use of two columns of different phases and lengths for orthogonal separation.
Modulation plays a central role in GC×GC performance. The high cost in ownership and operation of a commercial GC×GC system equipped with a cryogenic modulation motivated the development of cryogen-free modulation. In this study, a valve-based modulator based on the Deans switch served as an alternative to the commercial counterpart without the use of cryogen. The switching of an auxiliary gas stream of a Deans switch that cuts peaks from the column of first dimension (1D) into fine slices to the short column of secondary dimension (2D) created the effect of modulation.
Because of the low concentrations of VOCs in ambient air, usually at only sub-ppbv levels, an air sample would require substantial preconcentration before GC analysis. As a result, a self-built preconcentrator was connected to a GC×GC system, which was equipped with two columns of DB-1 (60 m×0.32 mm i.d.×1 μm d.f.) as the 1D column and Rtx-502.2 (2 m×0.32 mm i.d.×1.8 μm d.f.) as the 2D column to display the orthogonality of non-polarity vs. mid-polarity. Flame ionization detection (FID) was adopted by exploiting its high acquisition rates and reliability. Instead of using the commercial GC×GC software packages, a general-purpose software, Surfer®8, was used to plot GC×GC the results. The analytical precision of 7% as the RSD was achieved by repeated analysis of the PAMS standard gas mixture at sub-ppbv level. Ambient samples collected in a long highway tunnel by canisters and sorption tubes were analyzed for system validation. Furthermore, as the trial studies, PM2.5 aerosol samples collected on filter papers were attempted by thermal desorption of the filter paper. Compound identification was made with GC-MS by analyzing parallel samples to reveal the chemical identities of the major constituents of both the air and PM samples. While all the VOCs found in the canisters were non-polar hydrocarbons due to the lack of photochemistry in the tunnel, selected oxygenated VOCs (o-VOCs) were found in the aerosol sample owing to the extended oxidation process in the atmosphere.
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