激光光谱技术在溶解无机碳碳同位素分析中的应用

天然水溶解无机碳（DIC）碳同位素组成（δ13CDIC）分析是研究碳元素循环及相关生物地球化学过程的重要手段之一。近年来，激光光谱技术的发展为碳同位素比值测定提供了一种新的方法。文中阐述了一种总有机碳仪—激光光谱同位素仪联用在线测定水中DIC含量及δ13CDIC值的技术方法。该方法具有较高的测试精度，DIC含量测试结果相对标准偏差能控制在1%以内，δ13CDIC值精度优于±0.1‰（1σ）。不同类型岩溶水δ13CDIC值的测试结果与质谱仪法结果接近，差值总体≤0.3‰，表明该测试技术具有较高的准确度。由于吸收光谱信号与目标气体浓度有关，较低的CO2浓度会影响激光光谱仪的稳定性，在测试时需要根据DIC浓度控制样品进样量，最好采用多标样法来校准仪器测量值。激光光谱技术因其具有低成本、测试快速可靠，且仪器小巧便携等特点，在岩溶水溶解无机碳碳同位素分析中具有较大应用前景。 

Carbon isotopic analysis of dissolved inorganic carbon by laser spectroscopic technique

The analyses of the content of Dissolved Inorganic Carbon （DIC） in natural water and its stable carbon isotopic composition （δ13CDIC） provide an useful means for investigating the carbon cycle and its related biogeochemical processes. In recent years， the laser spectroscopic technology has provided us with a new method for isotopic analysis of DIC in water. In this study， we applied a TOC analyzer on-line with a laser Cavity Ring-Down Spectroscopy （CRDS） to determine DIC content and δ13CDIC value. The analytical precision and accuracy were discussed and both results determined by this system and the Isotopic Ratio Mass Spectrometer （IRMS） were compared. The results of the measurement of a large number of laboratory working standards and water samples with a wide range of DIC concentrations and δ13CDIC values show that such TOC-CRDS system has high analytical precisions. The relative standard deviation of DIC content test results are within 1%， and the precision of δ13CDIC is better than ±0.1‰ （1σ）. Measured δ13CDIC values by both TOC-CRDS and IRMS methods are close to each other， with a difference of mostly ≤0.3‰， an indicative of high accuracy for the TOC-CRDS system. Because the absorption spectrum is a function of gas concentration， however， the analytical precision and accuracy could be lowered for samples with lower CO2 yielding in the system. Thus， it is crucially important to make sure that the sample injected into the TOC analyzer has optimal carbon content. Moreover， it is better to use multiple working standards that have different δ13CDIC value to calibrate the measured values of water samples. Compared with the IRMS， the laser spectrometer is less costly and has advantages in fast， efficient determination of both DIC content and δ13CDIC value. In all， the machine is small and can be operated in the field， and it therefore has great potential to be one of useful methods in high-frequency monitoring of karst water， which benefits the study of karst dynamic system， karst critical zone processes and their responses to climatic and environmental changes.