Temperature issues in online extraction of water from plant and soil for stable isotope analysis
Temperature issues in online extraction of water from plant and soil for stable isotope analysis
复制标题
从植物和土壤中在线提取水进行稳定同位素分析的温度问题
DOI：
10.1002/rcm.8750
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发表时间：
2020
影响因子：
2
通讯作者：
Tian Lide
中科院分区：
文献类型：
--
作者：
Cui Jiangpeng;Tian Lide
低温真空蒸馏（CVD）和同位素比质谱（IRMS）是传统上用于从土壤和植物中提取和分析水的技术。1-3最近，在线提取和分析系统的发展，如感应模块腔衰荡光谱（IM-CRDS），4-6已经促进了从固体样品中快速提取水和每个样品在5-10分钟内进行同位素分析。然而，IM-CRDS和CVD-IRMS分析之间的一致性较差，4，6-8，原因尚未完全了解。我们目前对可能原因的了解包括有机污染，4，8样品的同位素异质性4和不完全提取。9萃取温度长期以来被认为是CVD的关键因素。10-13这是因为水萃取是一个瑞利蒸馏过程。一般来说，轻同位素已知首先被提取。温度对蒸馏的效率有关键影响，如果提取不完全，可能发生同位素分馏。此外，植物和土壤中都存在多个蓄水池。例如，土壤中含有四种水：重力水、毛细水、吸水水和结构水14、15;而植物组织中的水包括木质部、细胞间、细胞内、细胞壁和细胞器限制的水。16，17水的不同部分具有不同的同位素比率和对提取温度的敏感性。1，2，10，12，14，17随着提取温度的升高，提取的紧密结合水的分数升高。12，14在许多研究中已经评估了CVD中提取温度对同位素比率的影响，并且CVD中使用的温度通常接近100 C。1，3，15最近，推荐了更高的CVD提取温度（例如200 C），17，18特别是对于富含粘土的土壤。13，19然而，IM-CRDS的两个重要未知数如下：（1）不同加热方法下的浸提温度是多少？(2)不同的提取温度如何影响返回的同位素比率？迄今为止，Lazarus等人6的唯一报告发现，在IM-CRDS方法中，股骨柄的最高浸提温度约为133 ℃。在这封信中，我们报告了IM-CRDS中不同加热方法的提取温度的测量结果，并将其与CVD中广泛使用的提取温度进行了比较。我们还评估了提取温度对同位素比值的影响。IM-CRDS系统由L2130-i波长扫描腔衰荡光谱同位素水分析仪、感应模块和真空泵（均来自Picarro Inc.，美国加利福尼亚州圣克拉拉市）。IM-CRDS用户手册中的加热方法称为“配方”。20配方通过二阶多项式曲线控制感应加热的速率和持续时间：y= ax 2 + bx+ c，其中y是加热功率，x是时间（秒）。用户可以通过修改a、B和c的值以及加热持续时间来自定义加热方式。由于配方指定加热功率而不是温度，因此无法直接获得IM-CRDS系统中样品的浸提温度。在这里，我们测量了八种标准配方（低温、高温、叶片、厚叶、木质茎、粘土、粘壤土和桑迪壤土）和四种具有恒定加热功率的定制配方（constant_c_10、constant_c_15、constant_c_20和constant_c_40）的提取温度随时间的变化（每个配方的参数设置见图1标题）。在温度监测实验中，玻璃小瓶的底部被移除，并在玻璃小瓶的底部放置一个...
Cryogenic vacuum distillation (CVD) and isotope ratio mass spectrometry (IRMS) are the traditionally used techniques for the extraction and analysis of water from soil and plants. 1-3 Recently, developments in online extraction and analysis systems, such as induction module cavity ring-down spectroscopy (IM-CRDS), 4-6 have facilitated the rapid extraction of water from solid samples and isotope analysis in 5–10 min per sample. However, there is a poor agreement between IM-CRDS and CVD-IRMS analyses, 4, 6-8 and the reasons are not yet fully understood. Our current knowledge of possible causes includes organic contamination, 4, 8 isotopic heterogeneity of samples4 and incomplete extraction. 9 Extraction temperature has long been recognized as a key factor in CVD. 10-13 This is because water extraction is a procedure of Rayleigh distillation. In general, light isotopes are known to be extracted first. Temperature has a critical impact on the efficiency of distillation and isotopic fractionation may occur if extraction is not conducted completely. Furthermore, multiple water pools exist in both plants and soil. For example, soil holds four fractions of water: gravitational, capillary, hydroscopic and structural water14, 15; while water in plant tissue includes xylem, inter-cellular, intra-cellular, cell wall and organelle-constrained water. 16, 17 Different fractions of water have distinct isotopic ratios and sensitivities to extraction temperature. 1, 2, 10, 12, 14, 17 The fraction of extracted tightly bound water rises with an increase of extraction temperature. 12, 14 The impact of extraction temperature in CVD on isotopic ratios has been evaluated in many studies and the temperature employed in CVD is often near 100 C. 1, 3, 15 Recently, higher CVD extraction temperatures (eg 200 C) have been recommended, 17, 18 especially for clay-rich soils. 13, 19 However, two important unknowns with IM-CRDS are the following:(1) What are the extraction temperatures under different heating methods?(2) How do different extraction temperatures affect returned isotopic ratios? To date, the only report is by Lazarus et al6 who found that the maximum extraction temperature is about 133 C for stems in the IM-CRDS method. In this letter, we report on the measurements of the extraction temperatures of different heating methods in IM-CRDS and compare them with the extraction temperatures that are widely used in CVD. We also evaluate the impact of extraction temperatures on isotopic ratios. The IM-CRDS system consists of an L2130-i wavelengthscanned cavity ring-down spectroscopy isotopic water analyzer, an induction module and a vacuum pump (all from Picarro Inc., Santa Clara, CA, USA). The heating methods in the IM-CRDS user's manual are called “recipes.” 20 The recipes control the rate and duration of induction heating through a second-order polynomial curve: y= ax2+ bx+ c, where y is the heating power and x is the time (in seconds). Users can customize the heating method by modifying the values of a, b and c and the heating duration. Because the recipes specify heating power rather than temperature, one cannot directly obtain the extraction temperature of samples in the IM-CRDS system. Here we measured the changes in extraction temperature with time for eight standard recipes (low temperature, high temperature, leaves, thick leaves, woody stems, clay, clay loam and sandy loam) and four customized recipes with constant heating power (constant_c_10, constant_c_15, constant_c_20 and constant_c_40)(see Figure 1 caption for parameter settings of each recipe). In temperature monitoring experiments, the bottom of a glass vial was removed and a …