| 摘要: | 揮發性有機化合物 (Volatile Organic Compounds, VOCs)存在天然與人為排放源，除異味、毒性問題之外，亦可能因光化學反應產生二次污染如臭氧、懸浮微粒等空污問題，為大眾所關注的焦點。國內標準方法針對空氣中揮發性有機化合物已有採樣罐之方法。根據國內空氣污染防制法規定，於工業區緩衝地帶空氣品質監測設施標準中，若已有自動檢測方法，應優先以自動檢測方法為之，故現行之離線分析方法略顯不足，針對工業區緩衝地帶，開發線上連續監測空氣中揮發性有機化合物技術有其前瞻性與迫切性。
本研究成功開發一自動化線上VOCs連續監測方法，利用前濃縮儀搭配氣相層析儀 (Gas chromatography, GC)與質譜儀 (Mass spectrometry)進行連續偵測國內環檢所標準方法中NIEA A715.15B常見87種有害空氣污染物，考量台灣空氣相對濕度高對連續監測的質譜儀影響，建立最佳化條件。NIEA A715.15B標準品中77種VOC檢量線RSD值介於1.84~28.0%。而有害空氣污染物含鹵、醛、酮、酯等物質，物種複雜度高，又質譜儀針對輕碳物種感度不佳以及水氣干擾問題，故本研究亦利用heart cut技術與分流法，將輕碳VOCs (C1 ~ C4)引導至一吸附性管柱(PLOT)分離並以高靈敏度之鹵素偵測器 (Halogen specific detector, XSD)與火燄離子偵測器 (Flame ionization detector, FID)偵測；重碳VOCs (C5 ~ C12)則引導至一非極性管柱 (DB-1)分離後，以質譜偵測，達到各VOCs皆能有效的分離以及獲得良好的偵測靈敏度。
藉heart cut技術與多偵測器(質譜、FID、XSD)組成之系統，搭配適宜管柱進行二次分離，解決以往質譜只能使用單一管柱分離局部物質的限制，並大幅提高輕碳物質偵測靈敏度，同時獲取所有偵測物種濃度資訊，建立起空氣中VOCs連續監測技術，使未來更能即時鑑別物種濃度高值並了解特殊性工業區各工廠不正常排放對周界空氣品質的影響，擬訂法規管制排放以及維護國民健康。
;This study aims to develop an automated gas chromatographic (GC) technique measuring volatile organic compounds (VOCs) in ambient air. This on-line automated GC technique was based on the existing NIEA method A715.15B, which is an off-line method based on gas chromatography – mass spectrometry (GC-MS) developed by the Environmental Analysis Laboratory of Taiwan EPA. In this newly developed method, ambient air was first quantitatively enriched by drawing through a sorbent trap kept at sub-ambient temperatures without using cryogens, followed by thermal desorption (TD) to a GC equipped with multiple detectors, i.e., MS, flame ionization detector, and halogen specific detector (XSD) to analyze 87 target VOCs.
Because of the high humidity in ambient air and low sensitivity towards C1 - C4 species for mass spectrometry, both the heart-cut and flow-splitting techniques were used in GC configuration. The Deans switch cut the lighter effluents of C1 - C4 species from the DB-1 column to the PLOT for better separation. The C1 – C4 compounds were then simultaneously detected by both FID and XSD by splitting the effluent flow from the PLOT column for improved sensitivity and specificity, respectively. The remaining C5 – C12 spices from the DB-1 column were cut to MS in selected ion mode (SIM).
The performance of the automated GC method was assessed from the aspects of water management, concentration calibration, precision and detection limits for the adequacy of field applications in monitoring industrial emissions. The precision expressed by the relative standard deviation (RSD%) ranged from 0.39% to 6.67% based on repeated analysis of standard mixtures at sub-ppb levels. The precision for the calibration curves ranged from 1.84% to 28% for 77 out of the 87 target compounds.
Compared to the existing off-line method, the new on-line method successfully eliminated the need of cryogen and, at the same time, improved the sensitivity of C1- C4 compounds. The new method was then tested in the field for untended continuous monitoring of industrial pollutants. |
