揮發性有機化合物(Volatile organic compounds, VOCs)為臭氧與二次有機氣膠(Secondary organic aerosols, SOA)之前驅物，而多數VOCs物種在一般大氣環境中濃度為sub-ppb或~ppt (v/v)等級，低於多數氣相層析法(Gas chromatography, GC)所使用偵測器之偵測極限，因此在樣品導入GC作分析前，需使用裝載旋轉式六向閥(Six-port switching valve)的熱脫附儀(Thermal desorption, TD)，以化學吸附劑捕捉的方式進行樣品的前濃縮處理，惟長期使用下，旋轉閥本體與內部轉子在表面與氣流孔道所產生之磨損，往往造成氣流洩漏與橫竄等問題發生；除此之外，閥件系統中過多的管線接連處，使得無益體積(Dead volume)與額外管柱效應(Extra-column effect)造成了後端層析峰的拖尾。有鑑於此，本研究提出以無旋轉閥式的氣流導引熱脫附法(Flow-guided TD, FG-TD)解決上述閥件系統所面臨之問題。 FG-TD的進樣方式乃採Dean switch之配件Heart cut方法的精要，以三個微體積三通連接頭(Microvolume connector)與電磁三通閥所組成，藉由氣流的導引推動的方式達成樣品通過吸附管捕捉與熱脫附進樣，並有效地減少系統運作時所產生的磨耗。而與傳統六向閥為主的熱脫附儀作平行比對後，依舊能維持非常良好的表現，經由標準氣體測試，在高碳數分子(C6~C12)部分，對稱性由0.832提升至0.972，顯示在層析表現上獲得了良好的提升，且具有良好的再現性(RSD < 3 %)及線性關係(R2 > 0.99)，可作為未來濃縮儀進樣方式的選項之一。 ;Ambient volatile organic compounds (VOCs) are precursors of ozone and secondary organic aerosols (SOA). The concentrations of some of the common VOCs are usually at sub-ppb or even ppt(v/v) levels, rendering direct measurements below the detection limits of most detection methods for gas chromatography (GC). Using the thermal desorption (TD) method involving chemical sorbents and a 6-port switching valve provides a basic approach for sample pre-concentration prior to GC separation. However, the ware-and-tear of the rotor and the valve body after prolonged use often creates problems such as leakage, cross-channeling due to grooving, surface adsorption, etc. Excessive tubing connection around the switching valve also creates dead volume or the extra-column effect, resulting in peak tailing in GC analysis. In light of the aforementioned problems, a flow-guided TD device without using a switching valve is proposed and tested. The flow-guided TD device is derived from the Deans switch, which was configured by three micro-volume Tee connectors and solenoid valves. The sample flow going through the sorption tube for trapping and the carrier gas flow after TD for injection were made possible by directing the flows through different paths with a guiding flow to achieve minimum wear-and-tear as well as dead volume. Comparison with the conventional TD design was revealed by the improved peak symmetry for C6-C12 peaks from 0.832 to 0.972 on average, significantly alleviating the problem of peak tailing. The average precision for the new TD method was better than 3% and the linearity (R2) was greater than 0.99.
