Effect of preservation on the δ¹³C value of dissolved inorganic carbon in different types of water samples

Abstract

Recent studies have shown that the stable carbon isotope compositions of dissolved inorganic carbon (δ¹³C_DIC) of water samples preserved with HgCl₂ and CuSO₄ vary. Furthermore, mercury and cuprum compounds are toxic to the human or biological system and require proper waste disposal. To test the effect of preservation on the δ¹³C value of DIC in different types of water samples, a set of water samples with different DIC concentrations was preserved using different methods, including preserving with inhibitors (CuSO₄ or HgCl₂), preserving under frozen conditions, filtering through a 0.4 μ m paper filter, and the DIC species precipitated in the form of solid BaCO₃. Our results show that δ¹³C_DIC values of the samples preserved with CuSO₄ and HgCl₂ become more positive with increased storage time. The δ¹³C_DIC of the water samples preserved under frozen conditions and the precipitated DIC as BaCO₃ are also more positive than original water samples. However, the δ¹³C values were relatively stable for up to 90 days in all water samples filtered through the 0.4 μ m paper filter and stored under cool conditions (0–4 °C). Therefore, we suggest that the better method for the storage of water samples is to filter the samples through a 0.4 μ m paper filter while out in the field and preserve them under cool conditions, thereby avoiding the use of preservatives.

Keywords:

• carbon-13
• dissolved inorganic carbon
• isotope measurements
• methods and equipment
• preservation method
• storage time

Acknowledgements

We are greatly indebted to two anonymous reviewers for their valuable comments and suggestions. This study was financially supported by the National Science Foundation of China (nos. 41003011, 40873011) and West Light Foundation of the Chinese Academy of Sciences to X. Li.