Isotope ratio measurements of iron in blood samples by multi-collector ICP-MS to support nutritional investigations in humans

With the perspective of embarking on a human study using a double iron (Fe) stable isotope tracer protocol to assess iron bioavailability, investigations were conducted on Fe isotope ratios in blood samples using a VG Axiom Multi-collector ICP-MS. The factors affecting the precision and accuracy of Fe isotopic ratios, such as spectral- and matrix-induced interferences and Fe recoveries from sample preparation, have been identified and optimized. Major polyatomic interferences (e. g., Ar-O, Ar-OH, and FeH) were significantly reduced by using an Aridus nebulizer and desolvating system. Isobaric metal (e. g., Cr-54(+) on Fe-54(+) and Ni-58(+) on Fe-58(+)) interferences and Ca-oxides and hydroxides were quantitatively removed during chemical purification of blood samples and selective isolation of Fe by anion-exchange resin, after mineralization of the blood samples by microwave digestion. Quantitative recoveries of Fe from different steps of sample preparation were verified using whole blood reference material. Fe isotopic compositions of the samples were corrected for instrumental mass bias by the standard-sample bracketing method using the certified reference standard IRMM-014. External precisions on the order of 0.008-0.05 (% RSD), 0.007-0.015 (% RSD), and 0.03-0.09 (% RSD) were obtained for Fe-54/Fe-56, Fe-57/Fe-56, and Fe-58/Fe-56, respectively, in the blood for three replicate measurements. The level of precision obtained in this work enables the detection of low enrichments of Fe in blood, which is highly desired in nutrition tracer studies.