Measurement of a Doubly Substituted Methane Isotopologue, 13CH3D, by Tunable Infrared Laser Direct Absorption Spectroscopy

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• 作者：Shuhei Ono ; David T. Wang ; Danielle S. Gruen ; Barbara Sherwood Lollar ; Mark S. Zahniser ; Barry J. McManus ; David D. Nelson
• 刊名：Analytical Chemistry
• 出版年：2014
• 出版时间：July 1, 2014
• 年：2014
• 卷：86
• 期：13
• 页码：6487-6494
• 全文大小：396K
• ISSN：1520-6882

文摘

Methane is an important energy resource and significant long-lived greenhouse gas. Carbon and hydrogen isotope ratios have been used to better constrain the sources of methane but interpretations based on these two parameters alone can often be inconclusive. The precise measurement of a doubly substituted methane isotopologue, ¹³CH₃D, is expected to add a critical new dimension to source signatures by providing the apparent temperature at which methane was formed or thermally equilibrated. We have developed a new method to precisely determine the relative abundance of ¹³CH₃D by using tunable infrared laser direct absorption spectroscopy (TILDAS). The TILDAS instrument houses two continuous wave quantum cascade lasers; one tuned at 8.6 渭m to measure ¹³CH₃D, ¹²CH₃D, and ¹²CH₄, and the other at 7.5 渭m to measure ¹³CH₄. With the use of an astigmatic Herriott cell with an effective path length of 76 m, a precision of 0.2鈥?(2蟽) was achieved for the measurement of ¹³CH₃D abundance in ca. 10 mL STP (i.e., 0.42 mmol) pure methane samples. Smaller quantity samples (ca. 0.5 mL STP) can be measured at lower precision. The accuracy of the 螖¹³CH₃D measurement is 0.7鈥?(2蟽), evaluated by thermally equilibrating methane with a range of 未D values. The precision of 卤0.2鈥?corresponds to uncertainties of 卤7 掳C at 25 掳C and 卤20 掳C at 200 掳C for estimates of apparent equilibrium temperatures. The TILDAS instrument offers a simple and precise method to determine ¹³CH₃D in natural methane samples to distinguish geological and biological sources of methane in the atmosphere, hydrosphere, and lithosphere.