| 摘要: | 台灣環境部 (前環境保護署) 為了即時掌握臭氧前驅物的排放來源與了解物種與臭氧生成的關係，在全台設置十數個光化學評估監測站 (Photochemical Assessment Monitoring Stations, PAMS)，每站均針對特定的54種揮發性有機化合物 (Volatile Organic Compound, VOCs) 進行一小時一筆的連續監測。除了PAMS測站，環境部也開始針對有害空氣污染物 (Hazardous Air Pollutants, HAPs) 列表中之具毒性VOCs進行環境濃度監測。由於這些物種空氣濃度介於ppb (v/v) 甚至到ppt (v/v) 的低濃度範圍，需要熱脫附方法對空氣樣品進行濃縮。本研究承襲過去實驗室的經驗，開發一套具有低偵測極限熱脫附儀，並將其與主流之商業型GC/MS串聯以應用於環境監測。該儀器具備多項功能，包括管線溫度控制、漏氣測試、流量控制、複合進樣等，針對近百個VOC提供小時數據
最終開發的熱脫附儀經由台灣PAMS光化學評估監測站所參照的NIEA A505.12B標準方法進行效能驗證。測試結果顯示，儀器精密度的相對標準偏差值在1%至4.6%之間，符合標準方法規範的要求 (低於25 %)。儀器準確度測試顯示物種回收率介於84.48%至122.74%，符合標準規範範圍 (75 %-125 %)。此外，與光化測站現行使用的商業型熱脫附儀比較，本研究開發的儀器在分析圖譜中顯示出更好的峰分離效果，90%的峰具有較佳分離度，且所有峰均顯示較低的拖尾現象。
而在以質譜進行環境監測時，水氣對質譜離子源感度有顯著影響，離子源感度的快速下降會導致維護成本提高與數據連貫性不足等問題。本研究透過改變進樣條件－氣體樣品的濕氣程度，確認水氣是影響離子源感度的關鍵原因。實驗過程加入四種內標準品－Bromochloromethane, 1,4-Difluorobenzene, Chlorobenzene-d5, p-Bromofluorobenzene。內標準品可以校正質譜定量的濃度，也直觀地提供離子源感度表現。改變進樣條件並觀察四筆內標準品峰積分面積，來判斷離子源感度是否受影響。加入內標準品可以校正質譜定量的濃度，也直觀地提供離子源感度表現。經全乾氣體70筆測試後，四種內標準品總面積和為原先之103.7%；相對地，在第70筆加濕氣體測試後，其內標準品面積總和下降至僅剩55.10%。足見水氣對離子源壽命之影響。
在實際環境連續監測中，將普通層析管柱改為低流失 (Low bleeding) 管柱。低流失管柱低背壓、薄膜厚特性使其靜相較不易被沖刷出來到偵檢器，並導致離子源汙染而降低感度。研究結果顯示，低流失管柱測試後內標面積總和下降至47.49%；對比普通層析管柱面積總和下降至8.55%。透過分析結果可以推測管柱塗層與離子源感度之間的密切關係。使用低流失管柱能有效對抗水氣對於感度的影響，有助於延長離子源壽命，並提高監測儀器的穩定性與使用天數，減少維護成本。
;To promptly identify emission sources of ozone precursors and clarify the relationship between volatile organic compounds (VOCs) and ozone formation, the Ministry of Environment (former Environmental Protection Administration) established multiple Photochemical Assessment Monitoring Stations (PAMS) in Taiwan. These stations continuously monitor 54 VOCs hourly. In addition to PAMS, the Ministry of Environment also enacted regulations to monitor VOCs listed as hazardous air pollutants (HAPs). These toxic VOCs range from sub-ppb (v/v) to ppt (v/v), thus requiring the thermal desorption (TD) technique to preconcentrate air samples. This study builds on previous laboratory experience to develop a low detection limit TD system, coupled with a commercial GC/MS system for online monitoring. The instrument includes features such as temperature control, leak test, flow control, multi-sampling, etc.
The developed TD system was tested following the PAMS standard method, i.e., NIEA A505.12B. The precision test showed a relative standard deviation (RSD) of 1% to 4.6%, well below the method’s requirement of 25%. The accuracy test results demonstrated the recovery of 84.48% to 122.74%, withing the method’s criteria of 75% to 125%. When comparing the developed system with a commercial counterpart used at PAMS stations, 90% of the peaks showed improved separation, and all peaks displayed less tailing in chromatography.
Water vapor in the environment greatly affects the sensitivity of the mass spectrometer, leading to a rapid decrease in ion source sensitivity, increased maintenance costs, and inconsistent results. In this study, we demonstrate that moisture is the key factor affecting ion source sensitivity by varying the humidity of the sample and introducing four internal standards (Bromochloromethane, 1,4-Difluorobenzene, Chlorobenzene-d5, and p-Bromofluorobenzene) to evaluate the impact on ion source sensitivity. Internal standards allow for mass spectrometry calibration and direct monitoring of ion source stability. After 70 consecutive injections with dry gas, the peak areas of the four internal standards only changed by 3.7% on average. In contrast, after the 70th test with humid gas, the peak areas of the internal standard decreased to 55.10%, demonstrating the significant impact of moisture on the ion source’s sensitivity.
For environmental monitoring, we replaced regular capillary columns with low-bleed columns. These low-bleed columns, which feature low backpressure and thicker films, reduce the likelihood of stationary phase material being carried into the detector, thereby minimizing ion source contamination and maintaining sensitivity. Results showed that after testing with low-bleed columns, the internal standard area decreased to 47.49%, compared to only 8.55% left with regular columns. This result demonstrates how the properties of stationary phase can affect ion source sensitivity. Using low-bleed columns can effectively counteract the impact of water vapor on the ion source sensitivity, prolonging the service life of the ion source. Ultimately, this increases the stability and longevity of the monitoring instruments and reduces the overall maintenance costs. |
