Study on Influencing Factors of Dissolved Organic Carbon Isotope Determination in Water Samples of Karst Area by Wet Oxidation Method

The study examined the impact of different pure waters, headspace bottle atmospheric equilibrium methods, DOC content, and the removal of bicarbonate (HCO₃ ⁻ ) in water on δ¹³C_DOC values. The experimental results show significant differences in the ⁴⁴CO₂ peak areas produced by different blank pure waters, with the most prominent peak area being 3.8 times the smallest. Using a gas-tight syringe for atmospheric equilibrium can enhance the sample signal strength and ensure the optimal internal precision of the sample test results. Acidifying the water samples to pH＜3 Phosphoric acid can eliminate the influence of HCO₃ ⁻ in karst waters. A nitrogen-blowing apparatus at a 250mL/min flow rate for 5-10 minutes can remove the free CO₂ present after acidification. Under the above experimental conditions, the deviation between the test results and the calibrated reference values for karst water samples with DOC content greater than 2mg/L is better than 0.40‰ after blank correction. The deviation of δ¹³C_DOC in karst water samples measured by the wet oxidation method compared to the total organic carbon analyzer-stable isotope mass spectrometer online method is less than 0.30‰. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202405130108.