Development of a high-resolution ICP-MS method, suitable for the measurement of iron and iron isotope ratios in acid digests of faecal samples from a human nutrition study

High-resolution ICP-MS has been used to determine iron concentrations and isotope ratios in acid digests of faecal material from a human nutritional study (designed to investigate the absorption of iron from different types of diet). A resolution setting of 4000 was employed to separate the analyte isotopes from argon-based spectral interferences. Accurate correction of chromium and nickel isobaric overlaps required a correction for mass bias between the monitor isotopes 53Cr+ and 60Ni+, and the interfering isotopes 54Cr+and 58Ni+. Control faecal samples collected from each volunteer, and known to contain natural isotopic abundance iron, were used to calculate the mass bias correction factor, which was then applied to the remaining samples. The method was validated through the measurement of 190 separate acid digests of the certified reference material NIST 1577b (Bovine Liver), measured over a seven month period. The mean iron concentration of all 190 digests was 182 ± 16 µg g−1 compared to the certified value of 184 ± 15 µg g−1. Good agreement with the published natural 57Fe/58Fe isotope ratio was also observed, with a typical ‘in-batch’ precision of 1% RSD for the three replicates of NIST 1577b that were routinely analysed as part of each sample digestion batch. No significant difference was found between isotope ratios measured by ICP-MS (acid digested faecal samples) and by TIMS (fully cleaned up solutions originating from the same set of faecal samples). The preparation and analysis of samples by the ICP-MS method allowed a single analyst to digest, dilute and analyse 120 samples in duplicate in a period of 10 working days.