Dataset: Iodine speciation depth profile measurements from the Gotland Deep and Fårö Deep, Baltic Sea, collected aboard R/V Elisabeth Mann Borgese cruise EMB276 in Sep 2021

Seawater depth profiles for iodine analyses was collected onboard through HYDRO-BIOS free flow water samplers on a SBE32 water sampler which were mounted on a CTD rosette (conductivity-temperature-depth profiler). The samples were filtered through 0.2µm Sterivex^TM filters and collected in 125ml or 60ml amber Nalgene bottles once on deck and were frozen immediately afterwards for iodine preservation (Campos, 1997; Farrenkopf and Luther, 2002; Hou et al., 2001). Iodine speciation analyses occurred at Michigan State University within 1 year of sampling, mostly much sooner.

Glass columns for separating iodine species were washed by ultra-pure water produced from the Milli-Q^® water purification system, followed by packing with PYREX glass wool and 1ml of Bio-rad AG ^® 1-X8 anion exchange resin. During the chromatography procedure, the I^- fraction is specifically adsorbed onto the surface of the resin and eluted following the addition of a 2M HNO₃ (Fisher Scientific, TraceMetal^TM Grade)-18% (v/v) Tetramethylammonium hydroxide (TMAH, Fisher Scientific) solution. The eluent containing IO₃^- and DOI fractions were collected separately during the elution steps prior to the I^- collection, followed by an acidification (pH <2) and reduction step which converts iodine to I^- by the addition of concentrated hydrochloric acid (HCl, Fisher Scientific) and sodium metabisulfite (Na₂S₂O₅, VWR Analytical) (Hardisty et al., 2020). The treated eluent was left overnight for reaction and was processed through the same chromatographic procedure to collect the I^- derived from the IO₃^- and DOI fractions. For independent measurement of total dissolved iodine (TI) and as a quality control, a separate split of seawater samples was treated with HCl/ Na₂S₂O₅ to reduce iodine completely to I^- and subsequently processed through the chromatographic method. The eluents from chromatography were collected in Savillex^® PFA Teflon vials and then diluted 20 times in 0.2M HNO₃-0.9% TMAH before instrumental analysis. The iodine concentrations were measured using a Thermo Scientific iCAP Q triple-quadrupole inductively coupled plasma mass spectrometer (ICP-MS-TQ) using Qtegra software version 2.10.3324.131 in both single-quad (SQ) and triple-quad (TQ) modes with O₂ reaction cell gas. 

For quality control, a 200ppb I^- solution, which was gravimetrically diluted from the Inorganic Ventures© 1000±4ug/ml I^- ICP-MS standard solution in 1% tetraethyl ammonium (TEA), was  processed through chromatography alongside the seawater samples to estimate the yield of I^-, DOI and TI fractions. Similarly, for estimating the yield of IO₃^- reduction and column separation, a 200ppb KIO₃ solution dissolved from gravimetrically weighed solid KIO₃ powder (Fisher Scientific Education) was reduced and chromatographically processed. Yields from standards were applied to corrections for unknown samples. For monitoring contamination, ultra-pure water was processed through the columns. Among each set of seawater samples (11 samples) which were processed at the same time, one of them was randomly selected as a replicate. 

The eluents from the chromatography were collected in Savillex PFA Teflon vials and then diluted 20 times (0.2M HNO₃-0.9% TMAH) before instrumental analysis. The iodine concentrations were measured using a Thermo Scientific iCAP Q triple-quadrupole inductively coupled plasma mass spectrometer (ICP-MS-TQ) using Qtegra software version 2.10.3324.131 in both single-quad (SQ) and triple-quad (TQ) mode with O₂ reaction cell gas. The samples were introduced into the ICP-MS via a Teledyne Cetac ASX-520 autosampler. We used 5ppb internal standard solution diluted from 1001±3ug/ml indium (In) in 2% HNO₃, 999±5ug/ml Rhodium (Rh) in 15% HCl, and 1000±4ug/ml Cesium 0.1% HNO₃ (Cs) elemental standards from Inorganic Ventures© to calibrate the intensity of iodine and to correct for instrumental drift during the analytical run. The internal standard was either spiked into each sample or was introduced into the ICP-MS after being mixed with sample within an inline mixing chamber. Detection limits for samples were determined as 10 nmol/kg iodine.