Determination of Iodine in Carbonate Rocks by ICP-SFMS with NH₄HF₂ Digestion

Iodine in carbonate rocks is crucial for reconstructing the redox conditions of the ancient marine environment and tracing the sources of ore-forming fluids. However, it is difficult to determine the iodine in carbonate rocks due to the extremely low concentration (<1μg/g) and the loss of iodine-relative volatility in conventional acid digestion. In this study, a NH₄HF₂ digestion method was developed to prevent the loss of iodine in carbonate rocks during sample preparation. The sector field ICP-MS with high sensitivity was selected for the determination of iodine mass fractions in carbonate rocks. As results show, it took 1–6h at 220℃ for the complete recovery of iodine in carbonate rock GBW07114. The optimized mass ratio of ammonium bifluoride to the sample is 2, which avoided the reduction of sensitivity of iodine due to the introduction of a large amount of fluoride ions into ICP during the analysis of iodine in carbonate rocks. Moreover, ammonia dilution effectively eliminated the serious matrix effect induced by large amounts of Ca and Mg. The developed method was successfully applied to determine iodine in ten Chinese carbonate rock reference materials. The measured value of iodine in GBW07114 agreed with the certified value. Most of the measured values in carbonate rock reference materials agreed with their information values. The RSD values of analytical results were 3.3%–19.1%. There was a significant exponential correlation between the measured mass fraction and its RSD in these samples, which indicated that the measurement uncertainty was mostly due to the signal intensity of iodine. The high RSD values (15.0%–19.1%) of analytical results for GBW07128, GBW07130 and GBW07131 may be due to the heterogeneity of the samples. The LOQ of iodine for this method was 0.013μg/g, which satisfied the requirement for the determination of low content ioidne in carbonate rocks. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202505310138.