以重量法製備微量揮發性有機化合物標準氣體之研究

Preparation of trace VOC gas standard mixtures by gravimetric method

李雅琳(Ya-Lin Li)

指導教授 ： 王家麟

國立中央大學/理學院/化學系/碩士(2005年)

摘要

本研究利用自行發展之重量法製備濃度範圍約為ppm（v / v）之微量揮發性有機化合物標準混合氣體。此氣體內含沸點範圍介於-103.77℃~216.32℃間之C2-C12碳氫化合物，包括C2-C12之直碳鏈烷?，乙烯（ethylene）、丙烯（propylene）、1-丁烯（1-butene）以及1,3-丁二烯（1,3-butadiene）等烯?，以及苯（benzene）、甲苯（toluene）、鄰-二甲苯（o-xylene）、間-二甲苯（m-xylene）、1,2,4-三甲苯（1,2,4-trimethylbenzene）等芳香?類共計21個物種。
以毛細玻璃管裝盛此21種純物質後密封秤重，使用小數點第六位，單位為g之微量精密天平，秤重後計算其莫耳數，再將21隻毛細管裝入擊碎器並連接至稀釋系統，藉由稀釋氣體流洗至高壓鋼瓶內，配製成品濃度大約為 ppm（v/v）濃度範圍的標準混合氣體，共配製A、B兩罐高壓鋼瓶，最終壓力約為1000 psi（6.89 ×106 pa）。
隨後利用FID偵檢器的獨特特性－不同碳氫化合物之單位碳感度（response per carbon）皆大約相同的概念來對驗證成品A、B的回收率，評估此配製方法之可靠度。在回收率驗證中，以空管進樣方式，將一固定體積（約0.5 mL）與大約20 psi壓力的標準氣體導入氣相層析儀，針對C2-C5物質使用PLOT毛細管柱，C6-C12物質則使用DB-624管柱進行分析做回收率驗證。發現無論在A或是B中，C9以下之配製物種，單位碳感度均呈現良好之一致性，且配製之總不確定度經由計算得在A中為1.055％~1.870％，在B中則為1.250~2.215％。
與傳統重量法不同的是，在本研究當中，我們迴避使用高負載分析天平秤取配製成品鋼瓶重量而得到配製物種濃度，而是擬以內標的方法來訂出配製成品之最終濃度。本工作分為三個階段進行開發：1.系統與方法的建立；2.混合與回收率的驗證；3.內標的應用。
目前工作已完成配製、分析以及驗證方法之建立，而未來仍有待完成內標的開發與絕對濃度確立等工作。

關鍵字

單位碳感度 ； 回收率 ； 揮發性有機化合物標準氣體 ； 重量法 ； 內標準品

並列摘要

Standard gas mixtures containing 21 volatile organic compounds from C2-C12 at low parts per million level by volume (ppmv) was prepared by a gravimetric method developed in-house. Unlike conventional method with which a micro-balance is used for weighing minute amount of pure compounds and a large-scale balance is used for weighing filled cylinders, this under-developed method deliberately avoids the use of the large-scale balance owing to its high cost and inaccessibility to regular laboratories. To do so, one of the mixing compounds is chosen as the internal standard, dilution factor is then obtained by calibrating the internal standard with a commercially available single-compound standard, which is substantially less costly than purchasing a standard gas mixture.
21 target compounds were sealed in individual capillary tubes and weighed by a micro-balance accurate to 1×10-6 g before placing into a crusher connected to a 29.5 L evacuated aluminum cylinder and filling to approximately 1000 psi with high purity N2 as the balance gas. Dual cylinders of the same gas mixture were made to test for the reproducibility. Since the volatility of the 21 species vary drastically, the flushing from the crusher to the cylinder, termed “recovery” in this study, needs to be validated for all the species. A validation method based on equal-per-carbon-response as inherited by the flame ionization detector is applied. Gas chromatographic analysis of the gas mixtures confirmed an extremely uniform mixing for the more volatile compounds from C2-C8 as indicated by their highly agreeable per-carbon-response. For the other less volatile compounds from C9-C12 greater than expected values were obtained suggesting possible condensation near the cylinder head. Highly consistent results were observed between the two duplicate cylinders. Stability of the gas mixtures were tested by re-analyze the gas 5 months after the first analysis was performed, and no significant difference was found between the two analyses, proving the integrity of the mixture and applicability of the preparation method for compounds in C2-C8 range.

並列關鍵字

recovery ； internal standards ； gravimetric method ； gas standard mixtures of VOCs ； equal-per-carbon-response

參考文獻

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