Iodine stability as a function of pH and its implications for simultaneous multi-element ICP-MS analysis of marine carbonates for paleoenvironmental reconstructions

Abstract

Analyses of marine carbonates using solution-based ICP-MS typically occur in dilute acid, as alkaline solutions can cause CaCO3 and metal-oxide/oxyhydroxide precipitation. Measurement of iodine to calcium (I/Ca) ratios in marine carbonates—most often in the calcite shells, or tests, of foraminifera—are challenging to pair with those of pH-sensitive metals, as iodine is unstable in acidic solution and will volatilize in the presence of light and air. As a result, I/Ca ratios are routinely analyzed in basic solution, requiring additional sample material and analytical time. Leveraging the equilibrium relationships between thermodynamically stable, inorganic iodine species across a wide range of electrochemical potential (pE) and pH values, we determine which species are present in low-temperature, aqueous solutions characteristic of marine carbonate analysis on quadrupole ICP-MS, and suggest appropriate solution pH(s) for iodine measurement. Our results demonstrate that iodine is capable of volatilization at all pH values <10.5, though it is most volatile in the pH range of 0–1. Furthermore, at pH values above 10.5, solutions are susceptible to contamination from gas-phase molecular iodine, I2.Employing this knowledge of iodine speciation, we establish a high precision method for the solution-based analysis of iodine alongside twelve other elements (Li, Na, Mg, Al, Mn, Fe, Zn, Sr, Cd, Ba, U, and Ca) on a collision-reaction cell equipped quadrupole ICP-MS in the acidic pH range. Using a 9:1 volume/volume mixture of 0.5% tetramethylammonium hydroxide (TMAH) and 2% nitric acid (HNO3), we can reproducibly analyze iodine concentrations in a complex carbonate (calcite and aragonite) matrix alongside other elemental ratios used in paleo-environmental reconstructions. This mixture of dilute acid and base maintains acidic pH (1.5 ± 0.14), avoiding the precipitation of pH-sensitive metal oxides/oxyhydroxides and CaCO3 while minimizing iodine volatilization. We demonstrate that iodine concentrations measured using ICP-MS are only effectively stabilized in the pH range of 1.5–10.5 and recommend pH-matching sample and standard solutions. This method allows the extraction of considerably more directly comparable paleo-environmental information from the analysis of a single foraminiferal sample.