| 摘要: | 揮發性有機物質 ( VOCs ) 在環境中有許多不同層面的影響，因此在空氣中的定性、定量是評估VOCs問題的基本工作。可靠的濃度校正往往建構在標準氣體的使用，利用標準氣體穿插於連續量測過程的方法，稱之為外標校正 ( external calibration ) ，而內標校正 ( internal calibration ) 則是將標準品添加於樣品內，與樣品一同進入量測儀器。而作為良好的標準品必須具備化學成分輸出穩定的特性。市面上除了使用鋼瓶封裝的揮發性有機化合物，也有部分實驗室選擇使用市售的滲透管作為外標準品的來源，而對於高極性與高反應性物種的配製，使用動態的滲透管法配製較靜態的高壓鋼瓶來得恰當，然而此兩種方式價格均昂貴，且並不能保證長期的穩定性，故本研究擬開發一套成本低廉且製作方法簡便的滲透管外標法，並以內標準法驗證其可靠度，作為未來空氣中VOCs自動量測的線上校正方法。 本實驗建構一套能同時量測空氣中VOCs與氟氯碳化物 ( CFCs ) 的分析量測系統，模擬即時量測的情況，對鋼瓶內壓縮空氣進行連續監測，並於監測期間穿插市售標準品與自製滲透管樣品，以火焰離子偵測器 ( FID ) 偵測VOCs，電子捕捉偵測器 ( ECD ) 測量大氣中的CFC-113 ( CCl2FCClF2 ) ；由於CFC-113具有穩定存在於大氣中的特性，在此將其作為本系統的 「 原生內標 」 ，隨著VOCs一起被捕捉進樣，檢視連續量測期間內自製滲透管物質是否和內、外標維持一定比值，希冀藉由市售標準品與大氣中的原生內標CFC-113一同佐證自製滲透管作為線上校正方法的可能性。 本論文分為三階段實驗，第一階段使用原生內標CFC-113初步證明自製滲透管的輸出穩定性，CFC-113與自製滲透管的RSD值皆在4 % 內，兩者的感度相對變化皆在 ± 11 % 內；第二階段除了CFC-113之外，再額外加入VOCs混合標準氣體，藉由內、外標來體現自製滲透管作為外標的可行性，由結果顯示三者的RSD值皆在1.5 % 內，且感度相對變化落在 ± 5 %；由第二階段證明自製滲透管具有長時間穩定輸出的能力，具備作為外標的資格後，第三階段將其作為唯一的外標來源，搭配原生內標CFC-113於本校實地模擬野外測站之量測情景，將自製滲透管作為儀器系統偏移與量測物種濃度判斷之依據，自製滲透管與CFC-113在此次實地量測之RSD值均在2.5 %，感度相對變化皆在 ± 6 %，相較於內、外標的穩定，大氣中VOCs濃度跳動較為明顯，以Pentane為例，濃度範圍從0.1到4 ppbv，RSD值為82.0 %，感度相對變化為704 %。 本論文證明開發自製滲透管作為外標，提供線上校正的方法具有可行性，與市售標準品相比，除成本低之餘也具備可攜性，大幅降低了野外量測的不便度，未來本實驗室也將朝著填製反應性、極性較高等鋼瓶保存期限低的物種，以落實自製滲透管的使用價值。 Atmospheric volatile organic compounds (VOCs) are complex in composition and variable in abundance. To accurately assess their presence is qualitatively and quantitatively challenging. The reliable calibration of VOC concentrations often depends on the availability of robust standard gas mixtures. They are either analyzed by inserting in between sample aliquots in a continuous measurement process called external standardization, or blended into each sample aliquot called internal standardization. For both types of standardization, the gas mixtures must show high chemical stability overtime. For polar or reactive compounds, dynamic permeation method is preferred to static high pressure cylinders as the stable standard source. However, both types of standards are costly and the long-term stability cannot be ensured. As a result, this study attempted to develop a low-cost and simple external calibration method with self-made permeation tubes and a delivering system. Internal standardization employing a group of airborne compounds persistently existing in the atmosphere – chlorofluorocarbons (CFCs) – was also adopted for validating permeation tubes. In this study, we set up a heart-cut gas chromatographic system that can measure VOCs and CFCs simultaneously, by flame ionization detection (FID) and electron capture detection (ECD), respectively. The homogeneity of CCl2FCClF2 (CFC-113) in this study was used as an “intrinsic” internal standard to facilitate the validation of our self-made permeation tubes used for external standardization. The structure of this investigation was divided into three stages. At the first stage, we used atmospheric CFC-113 to validate the stability of the self-made permeation tubes. Both of them were smaller than 4% for the relative standard deviation (RSD), and the relative variations were less than 11%. At the second stage, both CFC-113 (internal standard) and a commercial VOC standard mixture (external standard) were used to assess the stability of the self-made permeation tubes. The results showed that the RSD for CFC-113, the VOC standard mixture, and permeation tubes was less than 1.5%, and the relative variations were less than 5%. Their comparable stability thus validated the self-made permeation method as an external standard source. At the last stage, selected VOCs in outdoor air were targeted continuously, for which the permeation method was the only source of external standardization for the two purposes of concentration calibration and system’s drift check. The RSD for permeation tubes and CFC-113 was less than 2.5%, and the relative variations were less than 6%. In contrast, the variability of VOCs in outdoor air was far more dramatic than that of CFC-113 and permeation tubes. For example, pentane in outdoor air varied between 0.1 and 4 ppbv, with an RSD of 82.0% and relative variation of 704%. Our results suggested that the self-made permeation tubes can be easily made and adopted as a stable calibration source. The light weight, simplicity, and low cost make this method highly desirable in field and lab applications. Future studies will include more polar and reactive VOCs to further broaden its applicability. |
