Determination of sub-nanomolar levels of iron in sea-water using reaction cell inductively coupled plasma mass spectrometry after Mg(OH)2 coprecipitation

A new procedure for the determination of iron in sea-water at sub-nanomolar concentration levels was developed. The method applied the low-blank magnesium hydroxide co-precipitation procedure in combination with quadrupole ICP-MS and used the dynamic reaction cell technique to resolve the polyatomic interferences arising from the residual matrix and the solvent. Although the interference-free determination at m/z 56 could be obtained by using methane as the reaction gas, the best signal-to-background ratio was achieved by using ammonia and performing the measurements at m/z 54. Accurate quantification of low levels of iron at m/z 54 also required the mathematical correction for the isobaric interference due to the occurrence of chromium. The main analytical figures of the optimized method were evaluated, with special attention to the determination of the procedural, reagent and field blanks. A detection limit as low as 0.09 nM was achieved, using a pre-concentration ratio of 10. The accuracy of the analytical procedure was evaluated by the analysis of the sea-water reference materials CASS-4, NASS-5, SAFe D2 and SAFe S1, having certified concentration values ranging from 12.8 nM to about 0.1 nM. The analytical precision for replicated analyses of the CRMs ranged from 1.3% to 14% (n=7). Finally, the developed procedure was applied to a number of open-ocean sea-water samples from the Ross Sea (Southern Ocean, Antarctica)