In situ carbon isotopic analyses of magnesium-rich carbonates by LA-MC-ICP-MS at low mass resolution using an...

Accurate in situ carbon isotopic measurements of magnesium-rich carbonates using LA-MC-ICP-MS require effective correction for isobaric interference, particularly from doubly charged Mg ions that can induce δ¹³C deviations of several permil. This study focuses on optimizing gas parameters and developing robust correction strategies to improve the accuracy of δ¹³C measurements. Experimental results demonstrate that the introduction of nitrogen gas at a flow rate of 4-6 mL/min effectively doubles signal sensitivity while optimizing low helium (0.1-0.2 L/min) and high argon (1.0-1.1 L/min) flow rates enhances the signal-to-noise ratio and minimizes Mg²⁺ interference. Three correction approaches were evaluated: (1) simple peak stripping without mass bias correction, (2) peak stripping with mass bias correction using bracketed standards, and (3) iterative mass bias correction. The simple peak stripping approach is limited to samples with low Mg/C ratios (²⁵Mg²⁺/¹²C⁺ ≤ 0.051 ‰). Bracketed standard calibration offers broader applicability but is affected by the instrumental mass bias factor (f_Mg), with long-term variations between 1.37 and 1.92 leading to δ¹³C deviations up to ∼2.35 ‰. This method performs well for carbonates with ²⁵Mg²⁺/¹²C⁺ ratios below 0.139 ‰, aligning closely with IRMS results, but introduces inaccuracies for dolomite and magnesite. The iterative correction approach yielded the most accurate δ¹³C values, consistent with IRMS results, with deviations within 0.38 ‰ and improved external reproducibility. This method negates the need for prior determination of f_Mg, offering enhanced external reproducibility. Utilizing optimal gas flow conditions and iterative correction, spatial resolution of 90 μm was achieved, with external reproducibility of 0.52 ‰ (2SD), representing a marked improvement over previous analytical methods.