Godwin Laboratory for Palaeoclimate Research, Department of Earth Sciences, University of Cambridge , Downing Street, Cambridge CB2 3EQ, United Kingdom.
Anal Chem. 2018 Jan 2;90(1):752-759. doi: 10.1021/acs.analchem.7b03136. Epub 2017 Dec 19.
We have developed a new method for measuring the isotopic composition (δO and δD) of different types of bonded water (e.g., molecular water, hydroxyl) contained in hydrated minerals by coupling a thermal gravimeter (TG) and a cavity ringdown laser spectrometer (CRDS). The method involves precisely step-heating a mineral sample, allowing the separation of the different types of waters that are released at different temperatures. Simultaneously, the water vapor evolved from the mineral sample is analyzed for oxygen and hydrogen isotopes by CRDS. Isotopic values for the separate peaks are calculated by integrating the product of the water amounts and its isotopic values, after correcting for background. We provide examples of the application of the differential thermal isotope analysis (DTIA) method to a variety of hydrous minerals and mineraloids including gypsum, clays, and amorphous silica (opal). The isotopic compositions of the total water evolved from a set of natural gypsum samples by DTIA are compared with the results of a conventional offline water extraction method followed by CRDS analysis. The results from both methods are in excellent agreement, and precisions (1σ) for δO (±0.12‰) and δD (±0.8‰) of the total gypsum hydration water from the DTIA method are comparable to that obtained by the offline method. A range of analytical challenges and solutions (e.g., spectroscopic interferences produced by VOCs in natural samples, isotopic exchange with structural oxygen, etc.) are discussed. The DTIA method has wide ranging applications for addressing fundamental problems across many disciplines in earth and planetary sciences, including paleoclimatology, sedimentology, volcanology, water exchange between the solid earth and hydrosphere, and water on Mars and other planetary bodies.
我们开发了一种新的方法，通过耦合热重分析仪（TG）和腔衰荡激光光谱仪（CRDS），测量含水矿物中不同类型结合水（如分子水、羟基）的同位素组成（δO 和 δD）。该方法涉及精确地逐步加热矿物样品，允许在不同温度下释放的不同类型的水分离。同时，通过 CRDS 分析从矿物样品中释放出的水汽的氧和氢同位素。通过积分水的量与其同位素值的乘积，并对背景进行校正，计算出各个峰的同位素值。我们提供了应用差热同位素分析（DTIA）方法于各种含水矿物和准矿物的例子，包括石膏、粘土和无定形二氧化硅（蛋白石）。通过 DTIA 从一组天然石膏样品中释放的总水量的同位素组成与传统离线水提取方法后进行 CRDS 分析的结果进行了比较。两种方法的结果非常吻合，DTIA 方法测量的总石膏水化水的 δO（±0.12‰）和 δD（±0.8‰）的精度（1σ）与离线方法相当。讨论了一系列分析挑战和解决方案（例如，天然样品中挥发性有机化合物产生的光谱干扰、与结构氧的同位素交换等）。DTIA 方法在解决地球和行星科学许多学科的基本问题方面具有广泛的应用，包括古气候学、沉积学、火山学、固-水圈之间的水交换以及火星和其他行星体上的水。
