Analysis of Influencing Factors and Results Correction for Carbon and Oxygen Isotope Measurement of Soil Carbonates by Continuous-Flow Isotope Ratio Mass Spectrometry

Continuous-flow isotope ratio mass spectrometry has been widely employed for the analysis of carbon and oxygen isotopes in carbonates, operating on the principle of introducing CO₂ generated from the phosphoric acid-carbonate reaction into an isotope ratio mass spectrometer for measurement. Water present in the phosphoric acid undergoes oxygen isotope exchange with the produced CO₂ at a temperature-dependent rate. Due to matrix differences between soil samples and pure carbonate reference materials, a systematic evaluation is required to assess their respective responses to phosphoric acid concentration and the feasibility of standard material calibration. This study utilized a multifunctional online gas preparation system (Iso FLOW) coupled with an isotope ratio mass spectrometer (IRMS) to systematically compare the effects of different phosphoric acid concentrations and reaction temperatures on the δ¹³C and δ¹⁸O values of carbonate reference materials and soil samples. The results demonstrated that: (1) for δ¹³C measurements, after reactions with 85%, 99%, and 100% phosphoric acid at corresponding temperatures of 70℃, 40℃, and 25℃ followed by standard material calibration, the δ¹³C values showed high consistency with a maximum deviation of 0.10‰, indicating negligible effects of phosphoric acid concentration and reaction temperature; (2) for δ¹⁸O measurements, δ¹⁸O values exhibited a progressively positive bias with increasing phosphoric acid concentration, with inconsistent bias patterns between carbonate standards and soil samples resulting in a maximum corrected deviation of 1.33‰, whereas the temperature effect could be effectively corrected by two-point calibration, maintaining δ¹⁸O deviation around 0.15‰. These findings indicate that phosphoric acid concentration significantly affects δ¹⁸O determination in soil carbonates, while the temperature effect can be effectively corrected. Therefore, the use of 100% phosphoric acid with a reaction temperature of 70℃is recommended to ensure both data reliability and analytical efficiency.