Exploratory Measurements of Singly Substituted and Clumped Isotopologues in Iodate Using Electrospray Ionization Mass...

Clumped isotope geochemistry can reveal the temperatures and chemical pathways at which molecules form, but it remains unexplored for most inorganic systems. Iodate (IO₃ ^-) is central to ocean and atmospheric iodine cycling and to tracking radioactive iodine from nuclear activity. Here, we report the first exploratory investigation into the feasibility of measuring bulk and clumped isotopologues in iodate using electrospray ionization mass spectrometry (ESI-MS).

Sodium iodate was dissolved in either deionized water or 97% H₂ ¹⁸O. Two sample sets systematically varied the H₂ ¹⁸O fraction of the spray solvent from 0% to 97%. Samples were infused into a Thermo Scientific TSQ Quantum Access MAX triple-quadrupole mass spectrometer with an atmospheric pressure ionization source operated in negative-ion scan mode. Spectra were acquired over m/z 110-190 and averaged across 25 scans.

The singly substituted isotopologue (¹²⁷I¹⁶O₂ ¹⁸O^-, m/z 177), clumped isotopologues (¹²⁷I¹⁶O¹⁸O₂ ^-, m/z 179; ¹²⁷I¹⁸O₃ ^-, m/z 181), and isotopologue distributions were resolved in IO₃ ^-. Relative isotopologue abundances shifted monotonically with solvent ¹⁸O content; ¹²⁷I¹⁸O₃ ^- increased from 0% to 56% across the enrichment range. In the natural abundance sample, measured isotopologue abundances matched the values predicted for a stochastic (random) isotope distribution, confirming that the instrument is sampling the natural isotopologue distribution without significant bias. We also report isotopologue distributions for IO^- and IO₂ ^-, which exhibit isotopic trends similar to those observed for IO₃ ^-.

These first measurements of isotopologues in IO₃ ^- establish ESI-MS as an accessible platform for isotopic analysis of inorganic oxyanions under enriched conditions. We discuss the thermodynamic basis for equilibrium clumping in IO₃ ^-, the analytical requirements for extending these enriched sample measurements to natural abundance clumped isotope thermometry, and the potential applications of this system in Earth and nuclear sciences.