Brady Matthew P, Hodell David A
Godwin Laboratory for Paleoclimate Research, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK.
Rapid Commun Mass Spectrom. 2021 May 30;35(10):e9078. doi: 10.1002/rcm.9078.
Oxygen and hydrogen isotopes are important tools for studying the modern and past hydrological cycle. Previous evaporation experiments used episodic measurement of liquid and/or vapor or did not measure all isotopologues of water. Here, we describe an evaporation experimental system that allows all isotopologues of liquid and water vapor to be measured simultaneously and near-continuously at high precision using cavity ring-down laser spectroscopy (CRDS).
Evaporating liquid is periodically sampled from a closed recirculating loop by a syringe pump that delivers a constant supply of water to the vaporizer, achieving a water vapor concentration of 20,000 ppmV H O (±132, 1σ). Vapor is sampled directly from the evaporation chamber. Isotope ratios are measured simultaneously with a Picarro L2140-i CRDS instrument.
For liquid measurements, Allan variance analysis indicates an optimum data collection window of 34 min for oxygen isotopes and 27 min for hydrogen isotopes. During these periods, the mean standard error is ±0.0081‰ for δ O values, ±0.0081‰ for δ O values, and ±0.019‰ for δ H values. For the derived parameters O-excess and d-excess, the standard error of the mean is 5.8 per meg and 0.07‰, respectively. For the vapor phase a 12.5 min data window for all isotopologues results in a mean standard error of ±0.012‰ for δ O values, ±0.011‰ for δ O values, and ±0.023‰ for δ H values. For the derived parameters, the standard error of the mean is 9.2 per meg for O-excess and 0.099‰ for d-excess. These measurements result in consistently narrow 95% confidence limits for the slopes of ln(δ O + 1) vs ln(δ O + 1) and ln(δ H + 1) vs ln(δ O + 1).
The experimental method permits measurement of fractionation of triple-oxygen and hydrogen isotopes of evaporating water under varying controlled conditions at high precision. Application of this method will be useful for testing theoretical models of evaporation and conducting experiments to simulate evaporation and isotopic equilibration in natural systems.
氧和氢同位素是研究现代及过去水文循环的重要工具。以往的蒸发实验采用间歇性测量液体和/或蒸汽的方式，或者没有测量水中所有的同位素变体。在此，我们描述了一种蒸发实验系统，该系统能够使用腔衰荡激光光谱技术（CRDS）同时且近乎连续地高精度测量液体和水蒸气的所有同位素变体。
通过注射泵从封闭的循环回路中定期采集蒸发的液体，该泵向蒸发器持续供应恒定的水量，使水蒸气浓度达到20,000 ppmV H₂O（±132，1σ）。直接从蒸发室采集蒸汽。使用Picarro L2140-i CRDS仪器同时测量同位素比率。
对于液体测量，阿伦方差分析表明，氧同位素的最佳数据采集窗口为34分钟，氢同位素为27分钟。在这些时间段内，δ¹⁸O值的平均标准误差为±0.0081‰，δ¹⁷O值为±0.0081‰，δ²H值为±0.019‰。对于导出参数¹⁷O-过量和d-过量，平均值的标准误差分别为每百万分5.8和0.07‰。对于气相，所有同位素变体的12.5分钟数据窗口导致δ¹⁸O值的平均标准误差为±0.012‰，δ¹⁷O值为±0.011‰，δ²H值为±0.023‰。对于导出参数，¹⁷O-过量的平均值标准误差为每百万分9.2，d-过量为0.099‰。这些测量结果使得ln(δ¹⁸O + 1) 对ln(δ¹⁷O + 1) 以及ln(δ²H + 1) 对ln(δ¹⁸O + 1) 的斜率的95%置信区间始终很窄。
该实验方法允许在不同的受控条件下高精度测量蒸发水的三重氧和氢同位素分馏。此方法的应用将有助于测试蒸发理论模型，并开展实验以模拟自然系统中的蒸发和同位素平衡。
