Measurement of a Doubly Substituted Methane Isotopologue, 13CH3D, by Tunable Infrared Laser Direct Absorption Spectroscopy
Measurement of a Doubly Substituted Methane Isotopologue, 13CH3D, by Tunable Infrared Laser Direct Absorption Spectroscopy
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DOI：
10.1021/ac5010579
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发表时间：
2014-07-01
影响因子：
7.4
通讯作者：
Nelson, David D.
中科院分区：
文献类型：
--
作者：
Ono, Shuhei;Wang, David T.;Nelson, David D.
甲烷是一种重要的能源和重要的长寿温室气体。碳和氢同位素比率被用来更好地限制甲烷的来源，但仅基于这两个参数的解释往往不能得出结论。通过提供甲烷形成或热平衡的表观温度，对双取代甲烷同位素(CH3D)-C-13的精确测量有望为来源特征增加一个关键的新维度。我们发展了一种利用可调谐红外激光直接吸收光谱(Tildas)精确测定(CH3D)-C-13相对丰度的新方法。Tildas仪器配备了两个连续波量子级联激光器；一个调谐到8.6微米，用于测量(CH3D)-C-13、(CH3D)-C-12和(CH4)-C-12，另一个调谐到7.5微米，用于测量(CH4)-C-13。使用有效路径长度为76m的像散Herriott池，在约10mLSTP(即0.42 mmol)的纯甲烷样品中，(CH3D)-C-13丰度的测量精度达到了千分之0.2(2sigma)。较小数量的样品(约0.5毫升STP)可以较低的精度进行测量。Delta(CH3D)-C-13测量的精确度为千分之0.7(2西格玛)，通过热平衡甲烷与一系列Delta D值进行评估。对于表观平衡温度的估计，千分之零点二的精度对应于25摄氏度时+/-7摄氏度和200摄氏度+/-20摄氏度的不确定度。Tildas仪器提供了一种简单而精确的方法来测定天然甲烷样品中的(CH3D)-C-13，以区分大气、水圈和岩石圈中甲烷的地质和生物来源。
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, (CH3D)-C-13, 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 (CH3D)-C-13 by using tunable infrared laser direct absorption spectroscopy (TILDAS). The TILDAS instrument houses two continuous wave quantum cascade lasers; one tuned at 8.6 mu m to measure (CH3D)-C-13, (CH3D)-C-12, and (CH4)-C-12, and the other at 7.5 mu m to measure (CH4)-C-13. With the use of an astigmatic Herriott cell with an effective path length of 76 m, a precision of 0.2 parts per thousand (2 sigma) was achieved for the measurement of (CH3D)-C-13 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 Delta(CH3D)-C-13 measurement is 0.7 parts per thousand (2 sigma), evaluated by thermally equilibrating methane with a range of delta D values. The precision of +/- 0.2 parts per thousand corresponds to uncertainties of +/- 7 degrees C at 25 degrees C and +/- 20 degrees C at 200 degrees C for estimates of apparent equilibrium temperatures. The TILDAS instrument offers a simple and precise method to determine (CH3D)-C-13 in natural methane samples to distinguish geological and biological sources of methane in the atmosphere, hydrosphere, and lithosphere.