水样中溶解性无机碳同位素测试前处理方法对比研究

水中溶解性无机碳(DIC)的碳同位素前处理方法由传统的BaCl2沉淀法,逐步发展到野外采样后直接通过连续流系统(GasBench-IRMS)测试.当前对于BaCl2沉淀法和连续流测试结果的差异,以及最佳DIC的前处理方法均需要开展深入研究.本文应用BaCl2沉淀法、医用无菌高密度聚乙烯瓶装样、GasBenchⅡ顶空样品瓶野外直接生成CO2气体三种前处理方法,对比研究前处理方法对采自桂林盘龙洞洞穴滴水和地下河水样中DIC碳同位素测试的影响.结果表明,由于BaCl2沉淀法使水样中游离CO2逸出,导致测定的DIC碳同位素值相对于另两种方法均偏正:地下河水样的碳同位素值最大偏正0.26‰;洞穴滴水的碳同位素值最大偏正0.33‰.因为野外水样采集环境的温度、大气压强与实验室内部环境变化较小,没有引起CO2、HCO3的溶解度改变,用医用无菌高密度聚乙烯瓶和GasBenchⅡ顶空样品瓶采集水样的碳同位素测试结果相同；相对而言,利用GasBenchⅡ顶空样品瓶直接产生CO2气体,能够避免外界环境条件变化导致CO2、HCO3-的溶解度发生变化引起碳同位素分馏,是精确测定水中DIC碳同位素最佳的前处理方法.

Contrastive Study of Sample-pretreatment Effects on Analytical Results of Inorganic Carbon Isotopes in Water Sample

The pretreatments for the measurement of dissolved inorganic carbon (DIC) isotopes in field water samples were developed from traditional BaCl₂ precipitation methods to directly continuous flow (GasBench-IRMS) methods. Further research needs be conducted to distinguish the difference between the traditional BaCl₂ precipitation methods and the continuous flow (GasBench-IRMS) method in order to select the best pretreatment for DIC. This study compared the results obtained by three pretreatment methods: BaCl₂ precipitation, medical sterile high-density polyethylene bottles and acidification of the sample in GasBench headspace vials in field work. Drip water and underground water samples were collected from Panlong cave in Guilin. The results show that because of CO₂ escape from the water sample, by using the BaCl₂ precipitation the value of the carbon isotope was more positive than other sample-pretreatment methods. The maximum deviation of the carbon isotope value from the underground river is 0.26. The maximum deviation of the carbon isotope value from the cave drip water is 0.33. The temperature and atmospheric pressure are similar in the field and the laboratory, therefore it did not cause solubility change of CO₂ and HCO₃^- in water samples. The DIC results of the medical sterile high-density polyethylene bottles and GasBench headspace vials are the same. The GasBench headspace vials pretreatment method can effectively avoid the solubility change of CO₂ and HCO₃^- caused by environmental changes, which leads to the CO₂ escape from the water sample or dissolved into the water sample from the atmosphere of CO₂. This change may cause carbon isotopic fractionation in DIC of water samples. Using GasBench headspace vials, which directly produce and collect CO₂ gas in the field is the recommended DIC pretreatment method.