不纯碳酸盐碳氧同位素组成的在线分析

Online analysis of carbon and oxygen isotopic compositions of impure carbonate

利用GVIsoPrimeⅡ型稳定同位素质谱仪测量不纯碳酸盐样品的碳氧同位素组成,这些样品是用国家碳酸盐碳氧同位素一级标准物质GBW04406与去除了碳酸盐的沉积物混合配制而成的,CaCO3含量在2%～90%之间。结果显示,δ13C内部精度为0.002‰～0.005‰(1σ),δ18O内部精度为0.003‰～0.009‰(1σ),与测量所得的纯CaCO3国际国内标准物质结果的内部精度范围一致,且外部精度达到仪器的指标要求,同时,不同CaCO3含量样品的δ13C和δ18O的测量值(测量平均值:δ13C=-10.932‰±0.021‰,δ18O=-12.483‰±0.054‰;1σ)也在误差允许范围之内与GBW04406推荐值(δ13C=-10.85‰±0.05‰,δ18O=-12.40‰±0.15‰;1σ)一致。可见碳酸盐的含量并不影响其碳氧同位素组成的分析结果,所以在线分析不纯碳酸盐的碳氧同位素组成是可行的,在线分析不纯碳酸盐样品的碳氧同位素组成之前应先对样品中碳酸盐含量进行大致估计,根据碳酸盐含量高低来确定样品用量以达到最佳分析效果。

The carbon and oxygen isotopic compositions of impure carbonate samples with CaCO3 content ranging from 2% to 90% are analyzed by GV IsoPrime Ⅱ stable isotope ratio mass spectrometry （IRMS） coupled with online carbonate preparing system （MultiPrep ）. The impure carbonate samples are prepared by mixing the national standard reference material for carbon and oxygen isotopes, GBW04406 （pure carbonate）, with some carbonate free sediments, which had been leached by HCl to remove carbonate. The analytical results demonstrate that the internal precision for δ^13C andδ^18O ratios is 0. 002‰-0. 005‰ （lσ） and 0. 003‰-0. 009‰ （1σ）,respectively for these impure carbonates, which is comparable to that for pure carbonates, such as the international and national standard reference material. As for the external precision, the analytical results of δ^13C andδ^18O ratios（mean values： δ^13C=- 10. 932‰ ±0. 021‰, δ^18O=-12. 483‰±0. 054‰; 1σ） of the samples with different CaCO3 contents are consistent with the reference values （δ^13C = - 10.85‰±0. 05‰, δ^18O= - 2.40‰ ±0. 15‰;1σ） of GBW04406 within analytical errors. Therefore, the content of carbonate has no effect on the analytical results of carbon and oxygen isotopic compositions. In conclusion, online analysis of the carbon and oxygen isotopes for impure carbonate is feasible. One point should be pay attention to impure carbonate analysis is to estimate the carbonate content of the sample beforehand, which may help to determine the best sample size and the acid volume to react with the carbonate for high quality results.