Seawater samples were collected at multiples depths in Niskin bottles at 4 stations ranging in distance from 40 to 530 km from Fukushima Dai-ichi nuclear power plant to be analyzed for levels of I129 and I127. The samples were stored in dark at ambient temperature before analysis.

0.5-1 L filtered seawater was transferred to a beaker; 125I solution (about 100 Bq, in NaI form) was added to the seawater. The prepared solution was then loaded to an anion exchange column (AG1-×4 resin, NO3- form, 1.0 cm in diameter and 15 cm length), the column was washed with 50 mL 0.2 mol/L NaNO3, the effluent and wash were combined for iodate separation. The column was then eluted with 150 mL 5% NaClO and then 50 mL of 3M HNO3, and the eluate was combined for iodide separation.

1.0 mL of solution of iodide fraction, iodate fraction and original seawater were taken into a vial for 127I measurement using ICP-MS. The remaining solution of iodate fraction (Efluent + washes of 0.2 mol/L NaNO3) or 200-500 mL of original seawater samples (for total 129I) was transferred to a beaker; 125I tracer (200 Bq), 0.5 mg 127I carrier, and 1M NaHSO3 solution were added. The solution was adjusted to pH 1-2 using 3 mol/L HNO3 to convert all iodine to iodide. The solution was then transferred to a separation funnel, 50 mL CHCl3 was added and then 1.0 mol/L NaNO2 solution was added to oxidize iodide to I2 to be extracted to CHCl3 phase by shaking. I2 is then back-extracted to the water phase by add 5 mmol/L NaHSO3 solution. This extraction and back-extraction steps were repeated once. The back-extracted aqueous phase was used for preparation of AgI target.

The remaining solution of iodide fraction was transferred to a separatory funnel. After addition of 0.5 mg of 127I carrier, 3.0 mol/L HNO3 was added to adjust pH1-2. 50 mL of CHCl3 and 5 mL of 1 mol/L NH2OH×HCl solution were added to reduce iodate to I2 to be extracted to CHCl3 phase by shaking. I2 in CHCl3 phase was then back-extracted using 5mM NaHSO3 solution.

0.5 mL of 1.0 mol/L AgNO3 solution was added to the back-extracted aqueous phases to precipitate iodide as AgI, which was separated using a centrifuge. The resulting AgI precipitate was dried at 70 °C and used for AMS measurement of 129I. 125I in the precipitate was counted using a NaI gamma-detector to monitor the chemical yield of iodine in the separation. Before extraction, the eluate of iodide from the anion exchange column was also measured for 125I by gamma-detector to monitor chemical recovery of iodide during column separation. This is used to correct 127Iodide and 129Iodide concentrations in seawater.

An ICP-MS system (X Series II, Thermo, Waltham, MA) equipped with an Xs- skimmer cone and standard concentric nebulizer was used for the measurement 127I. 1.0 mL of the separated fractions or the original seawater was diluted to 20 mL using 1% ammonium solution, and spiked with Cs+ (to 2.0 ppb) as internal standard. The detection limit of the method for 127I was calculated as 3 SD of the procedure blank to be 0.03 ng/mL.

The 129I/127I ratios in total iodine samples were determined by AMS at the Vienna Environmental Research Accelerator (VERA) and the University of Arizona AMS Laboratory, both using a 3MV National Electrostatics Corporation AMS. The 129I/127I ratios in iodide and iodate samples were measured using the 3 MV AMS facility at the Xi’an AMS center. The machine backgrounds of the 129I/127I ratio are around (2-4)×10-14. The blanks using the same procedure as the samples were also prepared; the highest measured 129I/127I ratio is 1´10-13, which is significantly lower than measured 129I/127I ratios in the samples.