Improving accuracy and precision of ice core δD(CH 4 ) analyses using methane pre-pyrolysis and hydrogen post-pyrolysis trapping and subsequent chromatographic separation
Improving accuracy and precision of ice core δD(CH 4 ) analyses using methane pre-pyrolysis and hydrogen post-pyrolysis trapping and subsequent chromatographic separation
复制标题
利用甲烷预热解和氢气热解后捕集以及随后的色谱分离提高冰芯 δD(CH 4 ) 分析的准确性和精密度
DOI：
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发表时间：
2013
期刊：
影响因子：
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通讯作者：
H. Fischer
中科院分区：
文献类型：
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作者：
M. Bock;J. Schmitt;J. Beck;R. Schneider;H. Fischer
抽象的。冰雪和极地冰芯提供了唯一直接的古大气档案。分析过去的温室气体浓度及其在冰层气泡中的同位素组成有助于限制过去全球生物地球化学循环的变化。为了分析甲烷(δD(CH4)或δ2H(CH4))的氢同位素组成，迄今使用了0.5至1.5千克的冰。本文提出了一种提高冰芯空气中甲烷D(δD，CH4)测量精度和减少采样量的方法。预先浓缩的甲烷集中在高温炉前(预热分解捕集)，随后热解形成的分子氢被捕集(热解后捕集)，两者都在−196°C的碳图毛细管上。热解前和热解后的捕集在很大程度上消除了在色谱分离过程中产生的同位素分馏，并在质谱仪中产生了一个狭窄的峰。空气标准的测量精度可以优于1‰。对于冰川时期的极地冰样，我们估计350ppb甲烷含量的350g冰(在标准温度和压力下约为30毫升空气)的精度为2.3ppb(‰)。这相当于最近的对流层空气样本(约1900 ppb CH4)，约为6毫升(STP)或约500 pmol的纯CH4。
Abstract. Firn and polar ice cores offer the only direct palaeoatmospheric archive. Analyses of past greenhouse gas concentrations and their isotopic compositions in air bubbles in the ice can help to constrain changes in global biogeochemical cycles in the past. For the analysis of the hydrogen isotopic composition of methane (δD(CH4) or δ2H(CH4)) 0.5 to 1.5 kg of ice was hitherto used. Here we present a method to improve precision and reduce the sample amount for δD(CH4) measurements in (ice core) air. Pre-concentrated methane is focused in front of a high temperature oven (pre-pyrolysis trapping), and molecular hydrogen formed by pyrolysis is trapped afterwards (post-pyrolysis trapping), both on a carbon-PLOT capillary at −196 °C. Argon, oxygen, nitrogen, carbon monoxide, unpyrolysed methane and krypton are trapped together with H2 and must be separated using a second short, cooled chromatographic column to ensure accurate results. Pre- and post-pyrolysis trapping largely removes the isotopic fractionation induced during chromatographic separation and results in a narrow peak in the mass spectrometer. Air standards can be measured with a precision better than 1‰. For polar ice samples from glacial periods, we estimate a precision of 2.3‰ for 350 g of ice (or roughly 30 mL – at standard temperature and pressure (STP) – of air) with 350 ppb of methane. This corresponds to recent tropospheric air samples (about 1900 ppb CH4) of about 6 mL (STP) or about 500 pmol of pure CH4.