Sensitive and rapid determination of the iodine/calcium ratio in carbonate rock samples by ICP-MS based on solution cathode glow discharge sampling†

The iodine-to-calcium ratio (I/Ca) in carbonate rocks has been extensively used to indicate marine oxidation states. However, the low-iodine and high-calcium characteristics of carbonate samples pose challenges in achieving rapid and accurate determination of I/Ca. In this study, we have developed a solution cathode glow discharge (SCGD) sampling technique coupled with inductively coupled plasma mass spectrometry (ICP-MS) for highly sensitive and accurate determination of I/Ca in low-iodine carbonate samples. The proposed method takes full advantage of the high-efficiency vapor generation of iodine and the inefficient introduction of calcium by SCGD, thereby significantly enhancing the sensitivity for iodine measurement and reducing matrix interference. In addition, it enables the simultaneous determination of I and Ca even in high-calcium samples. Compared to conventional pneumatic nebulization sampling, the SCGD sampling method demonstrated a 100-fold sensitivity improvement for iodine determination, while Ca sensitivity was reduced by a factor of 70. The effect of operating parameters and reaction conditions on the signal intensities was investigated. Under optimized conditions, the iodine detection limit was as low as 0.9 pg g⁻¹ for carbonate samples. Finally, the validity of the method was verified through the analysis of standard reference samples, actual carbonate rocks, and simulated samples. The results conclusively demonstrate that our developed method offers a simple, sensitive, and rapid approach for accurately measuring the I/Ca ratio in carbonate rocks, thereby facilitating broader application of the I/Ca indicator.