| 摘要: | 揮發性還原性硫化物(volatile reduced sulfur compounds, RSCs)普遍存在於大氣中，其嗅覺閥值甚低(數個ppbv)，容易造成環境臭味問題而影響生活品質，此外對全球大氣也扮演著氣候效應(climate effect)的角色。本研究的目的是希望建立或改良分析周界RSCs如H2S、C2H5SH、CH3SCH3、CS2及CH3SSCH3的方法；由於RSCs濃度低、反應性高、生命期短、在分析設備的管線或採樣容器內會有易沾黏的特性，其中又以H2S最易受水氣及氧化物反應而損失，更增加了檢測的難度。周界RSCs濃度低至ppbv至 pptv，往往因濃度低於偵測器偵測極限而不利偵測，故本次研究企圖尋找一適合吸附材料，佐以降溫，將RSCs先行濃縮後，再熱脫附進入氣相層析儀將物種分離，偵測器選用對硫元素靈敏性佳之脈衝式火焰光度( pulsed flame photometric detector, PFPD )。 現今最常用以捕捉RSCs的吸附材料為Molecular sieve 5A及Tenax TA，Molecular sieve 5A主要捕捉H2S，Tenax TA則適合捕捉其他硫化物。本次研究希冀能找到單一材料捕捉所有常見的RSCs，並且在效率上優於以上材料。經嘗試以微孔( < 20Å )、中孔( 20Å-500Å )、大孔洞( >500Å )等諸多孔洞材料觀察脫附溫度曲線，發現Unibeads 1S不僅脫附溫度曲線表現良好且捕捉效率明顯優於其他材料，捕捉H2S偵測極限可低至0.13 ppbv，RSD值為 1.78%，檢量線線性高達0.999以上，明顯優於Molecular sieve 5A，而捕捉C2H5SH的效率也優於Tenax TA，在CS2、DMS及DMDS表現上則和Tenax TA相近，C2H5SH、DMS、CS2及DMDS的RSD值皆低於1.76%且線性皆0.995以上，其中DMS、CS2及DMDS偵測極限皆低至0.11 ppbv以下，因此使用Unibeads 1S單一材料即可取代現今常用之Molecular sieve 5A、Tenax TA作為RSCs的最佳線上濃縮介質，並且以此吸附劑作為前濃縮介質並搭配自動化前濃縮系統而成功應用於線上(on-line)及離線(off-line)之RSCs實地量測。Reduced sulfur compounds (RSCs) are commonly found in ambient air, which easily give off foul odor in the environment at very low concentrations, thus affecting the quality of life for residents near the sources. In addition, RSCs also have significant implications in global climate change. The objective of this research is to develop a GC technique to analyze ambient H2S, C2H5SH, CH3SCH3, CS2, and CH3SSCH3 of RSCs using pulsed flame photometric detector (PFPD). Common challenges in the analysis of ambient RSCs include: 1. low concentration detection, 2. sample integrity due to high reactivity, 3. wall effect arising from the “stickiness” of RSCs on tubing or container’s inner surface, 4. the loss of H2S to water or oxidation, etc. Ambient RSCs are often found at levels from ppbv to pptv, which is below the detection limits of a GC detector. As a result, a preconcentration step is needed prior to GC analysis to allow quantitative analysis. The performance of preconcentration relies largely on the proper selection of sorption materials. In this research, several materials tested at various trapping temperatures for the low levels of RSCs. The Taiwan EPA method (NIEA A701.11C) uses Molecular Sieve 5A and Tenax TA to preconcentrate H2S and other sulfur compounds, respectively. This study is aimed at finding a single suitable sorption material which can enrich all RSCs within each sample aliquot with improved efficiency, manifested by better precision, linearity and lower limits of detection (LOD). The pore sizes of the test materials ranged from microporous (< 20Å) to macroporous ( >500Å ). Of the 17 materials tested, it was found that Unibeads 1S out-performed the rest in all criteria of quality assurance. For instance, the LOD is as low as 0.13 ppbv, the RSD is 1.78%, and the linearity is 0.999 (R2) for H2S, suggesting Unibead 1S is better than Molecular sieve 5A in H2S preconcentration. Unibeads 1S also showed better efficiency than Tenax TA for C2H5SH, and comparable performance for CS2、DMS and DMDS. With Unibeads 1S, the RSD for C2H5SH、DMS、CS2 and DMDS is better than 1.76%, the linearity is greater than 0.995, and the LOD (3σ) for DMS、CS2 and DMDS is below 0.11 ppbv. As a result, Unibeads 1S can replace Molecular sieve 5A and Tenax TA as the better choice for online preconcentration of ambient RSCs. Having proven the performance of Unibeads 1S, a thermal-desorption (TD) GC-PFPD was used to on-line monitor the ambient RSCs. Grab samples collected from potential sources of RSCs were also analyzed by TD GC-PFPD and the results were reported. |
