| 摘要: | 全面二維氣相層析 (Comprehensive two-dimensional gas chromatography，GC×GC)技術是分離複雜揮發性有機化合物 (Volatile organic compounds，VOCs)的最佳選擇，與傳統一維氣相層析 (conventional one-dimensional GC)相比，大幅提升其峰容量，將樣品全部通過兩支不同作用力、長度之管柱，使在一維圖譜中共析的物種能藉由第二種作用力被分離開來。當與時間飛行質譜儀 (time-of-flight mass spectrometer，ToF)結合，高度複雜樣品如汽油、香水、香味、環境樣品等即可利用GC×GC-ToF妥善分離樣品並定性未知物，在檢測上具有強大的能力。
調制器在GC×GC中扮演重要的角色，在商業化GC×GC系統中常搭配冷凍型調制器，設備成本高且冷劑開銷大，故想開發一套不需使用冷劑的調制器。本篇研究中選用Deans switch作為閥件型調制器，藉由輔助氣流的轉向，將一維管柱層析峰切片進樣至較短的二維管柱作分析，達到調制的效果，也避免商業化調制器中冷劑的開銷，降低實驗成本。
由於分析樣品在大氣中為ppb等級，須經由前濃縮再進樣至GC分析，實驗系統為前濃縮儀連接GC×GC系統，以DB-1及Rtx-502.2分別為一維、二維管柱，以沸點對極性展現正交性，透過火焰離子偵測器 (FID)的高資料擷取速率得到可靠的實驗數據，再以商業化Surfer®8軟體繪製二維圖譜。系統穩定性以ppb等級之PAMS標準品連續進樣7次，得到RSD值約7%，顯示系統足以穩定分析。真實樣品分析採集自空氣流通不佳隧道之採樣罐及採樣管樣品，驗證系統最佳化，另外，初次針對PM2.5氣膠樣品，以石英濾紙吸附再加以熱脫附送入分析系統，再輔以實驗室GC-MS對採樣樣品及氣膠樣品平行比對，可成功分析隧道樣品及氣膠樣品中的化學組成。在隧道樣品中，物種缺乏光化反應，故看到物種多為非極性的碳氫化合物；而氣膠樣品在大氣中旅行中多已額外被氧化，測出物種大多數為含氧VOCs (o-VOCs)。
;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. |
