State Key Laboratory for Mineral Deposits Research and Center for Marine Geochemistry Research, Department of Earth Sciences, Nanjing University, Nanjing, 210093, PR China.
Rapid Commun Mass Spectrom. 2012 Apr 15;26(7):805-10. doi: 10.1002/rcm.6164.
Precise measurement of the carbon isotopic compositions of dissolved inorganic carbon (DIC) in water samples is very important for earth and environmental sciences, and the H(3) PO(4) acidification method has long been applied for this purpose. Due to the large variation in DIC concentration in various types of water samples, however, it is difficult to sample the optimized amount of water for high-precision mass spectrometry analysis and in many cases this has led to poor data quality. In addition, when the amount of water sample is very limited or when the water has a complicated composition such as seawater and saline marine pore waters, it is often difficult to obtain high-quality data using the conventional method.
Here we report a new method of obtaining CO(2) -water equilibrium for high-precision carbon isotope analysis of DIC in water samples. The instrument used was a Delta Plus XP stable isotope mass spectrometer coupled with an on-line a Gas Bench II sample preparation device. In general, 0.5 mL of water is sampled and equilibrated with CO(2) in headspace. The CO(2) is then transferred into the mass spectrometer and δ(13) C values are obtained.
The δ(13) C values for four marine pore water samples determined by this new method are only slightly different (<0.15‰) from those determined using the conventional H(3) PO(4) method. The new method of obtaining CO(2) -water equilibrium also allows a constant amount of CO(2) for every sample to finally enter the mass spectrometer and hence better precision can be achieved.
This new method is suitable for the measurement of both oxygen isotopic composition in water and carbon isotopic composition of DIC for seawater and pore water samples. Another benefit of this new method is that the water sample can be re-used for other further analyses and this is critical when the amount of sample is limited.
精确测量水样中溶解无机碳（DIC）的碳同位素组成对于地球和环境科学非常重要，长期以来，H（3）PO（4）酸化法一直用于此目的。然而，由于各种类型水样中 DIC 浓度的巨大变化，难以采样优化量的水进行高精度质谱分析，在许多情况下，这导致了数据质量较差。此外，当水样量非常有限或水样组成复杂，如海水和咸水海洋孔隙水时，通常很难使用常规方法获得高质量的数据。
本文报道了一种新的方法，用于获得水样中 DIC 的高精度碳同位素分析的 CO（2）-水平衡。所用仪器为与在线 GasBench II 样品制备装置耦合的 Delta Plus XP 稳定同位素质谱仪。通常，采集 0.5mL 的水样并与顶空 CO（2）平衡。然后将 CO（2）转移到质谱仪中并获得 δ（13）C 值。
用这种新方法测定的四个海洋孔隙水样的 δ（13）C 值与用常规 H（3）PO（4）法测定的结果仅略有差异（<0.15‰）。获得 CO（2）-水平衡的新方法还允许每个样品的 CO（2）量恒定，最终进入质谱仪，从而可以实现更好的精度。
这种新方法适用于海水和孔隙水样中水中氧同位素组成和 DIC 碳同位素组成的测量。这种新方法的另一个好处是可以重新使用水样进行其他进一步的分析，当样品量有限时，这是至关重要的。
