| 摘要: | 存在於大氣中的揮發性有機化合物(Volatile Organic Compounds, VOCs)本身是空氣汙染物，也是形成臭氧及二次有機氣溶膠的前驅物。VOCs在一般大氣環境下的濃度很低(~ppb或~ppt等級)，如何線上濃縮VOCs是氣相層析(GC)進樣的關鍵步驟。
本研究以自製不須冷劑降溫之熱脫附(Thermal Desorption, TD)前濃縮方法，嘗試在活性碳、矽及鋅等多孔洞材料中，利用其比表面積、孔徑體積及孔徑大小等不同特性，找到一個良好的前濃縮介質取代參考之3 in 1配方，並藉由前濃縮介質本身之吸脫附特性差異，尋找可獲得最佳熱脫附峰形之材料，以提升VOCs之定性及定量工作。
由於乙烷、乙烯、丙烷、丙烯等具高揮發性的物質，容易在濃縮時貫流(breakthrough)，欲捕捉這類小分子物質，除了降溫之外，濃縮介質本身的孔徑特性也很重要。在吸脫附測試過程中，發現矽及鋅材料對這類物種的捕捉效率差，而活性碳材料對VOCs則有較佳的捕捉能力，推測是因為活性碳材料具微孔特性(平均孔徑大約2 nm)以及高比表面積(~3000 m2g-1)，然而即使捕捉溫度已降到-40℃，乙烷、乙烯仍有貫流現象，因此在前端加入一個對輕碳物種具有良好捕捉能力的Carbosieve S?，形成雙重床之捕捉管，當TD降溫至-20℃，即可完全捕捉C3-C12之VOCs物種，當其降溫至-40℃便能有效捕捉C2物種，解決C2物種貫流的問題，且具有良好的再現性(RSD < 4%)以及線性關係(R2> 0.99)。
由於層析峰形好壞會影響定性及定量之工作，而峰形又與濃縮介質對VOCs物種之熱脫附特性有關，在熱脫附峰形之測試結果中，以活性碳材料之熱脫附峰形表現最好，因其峰形較窄高且對稱性較佳，但仍有拖尾現象，因此利用中心切割(heart-cut)技術將熱脫附峰進行切片分析，診斷出造成拖尾之物種為重碳物種，雖然藉由分流可改善拖尾現象，並提高熱脫附峰之對稱性，但仍進一步調整系統參數加以改良熱脫附峰形，這些參數包括GC端之載流氣體流速、烘箱升溫程序，以及TD端之線圈加熱最大輸出功率、改變加熱方式，最終得以藉由增快升溫速度，改善並獲得較對稱之熱脫附峰形。
Ambient volatile organic compounds (VOCs) are not only toxic at high concentration, but also act as precursors of ozone and secondary organic aerosols (SOA). Monitoring ambient VOCs often requires the step of pre-concentration prior to gas chromatographic (GC) analysis. Sorbent adsorption and thermal desorption (TD) in the process of pre-concentration inevitably result in a certain degree of peak tailing and asymmetry affecting qualitative and quantitative results.
In this study, we used a self-built cryogen-free TD device to test for a series of sorption materials including activated carbons, mesoporous silicates, and Zn porous materials. Special attention was paid to ethane, ethylene, propane, and propylene because of their extremely low boiling points, which easily results in pronounced breakthrough problem and hence low recovery. Of all the tested porous materials, the activated carbon materials were found to exhibit better performance than Si and Zn materials. Although they can trap the widest range of VOCs, but still showed insufficient efficiency on C2 compounds, even trapping at low temperature (-40℃) and high inlet pressure (40 psi). As a result, the commercial Carbosieve S?, which is a microporous sorbent, was added in the sorbet bed to improve the C2 recovery.This dual sorbent formulation of activated carbon materials plus Carbosieve S? was able to effectively trap the full range of C2-C12 compounds with desired linearity (R2> 0.99) and reproducibility (RSD < 4%).
The use of activated carbon materials as sorbents also produced much narrower and more symmetric peaks than the the use of other materials. Even so, slight peak tailing still existed. To diagnose the cause of tailing, the Deans swtich method was adopted to slice the desorption peak for the difference in composition. Compound discrimination were found between different slices and higher boiling VOCs tended to reside more in the later slices than earlier ones.
In addition to the aforementioned efforts of sorbent selection and TD peak diagnosis, other TD variables, such as the TD heating rate and flow splitting, were also tuned to optimize peak shape and symmetry. |
