Method development for the redox speciation analysis of iron by ion chromatography–inductively coupled plasma mass spectrometry and carryover assessment using isotopically labeled analyte analogues

An ion chromatography–inductively coupled plasma mass spectrometry (IC-ICP-MS) method was developed for the redox speciation analysis of iron (Fe) based on in-column complexation of Fe2+ and Fe3+ by dipicolinic acid (DPA). The effects of column type, mobile phase composition and molecular ion interference were studied in the method optimization. The carryover of the target species in the IC-ICP-MS method was uniquely and effectively evaluated using isotopically enriched analogues of the analytes (54Fe2+ and 57Fe3+). Standard solutions of the enriched standards were injected into the system following analysis of a sample, and the ratios of the isotopes of iron in the enriched standards were calculated based on the chromatographic peak areas. The concentrations of the analytes carried over from the sample to the enriched standards were determined using the quantitative relationship in isotope dilution mass spectrometry (IDMS). In contrast to the routine way of evaluating carryover effect by injecting a blank solution after sample analysis, the use of isotopically enriched standards identified significant analyte carryover in the present method. Extensive experiments were carried out to systematically identify the source of the carryover and to eliminate the problem; the separation column was found to be the exclusive source. More than 95% of the analyte carryover was eliminated by reducing the length of the column. The detection limit of the IC-ICP-MS method (MDL) for the iron species was 2 ng g-1. The method was used to determine Fe2+ and Fe3+ in synthetic aqueous standard solutions and a beverage sample.