Mass spectrometric analysis Preparation of blood samples for mass spectrometric analysis and preparation of enriched standards will be followed as per the procedures described by Kastenmayer et al. Samples of heparinized whole blood will wet ashed with concentrated HNO3 at a ratio of 1:3 (blood: acid)(vol/vol) on a hot plate over low heat for 20-30 min. Digested solutions will be adjusted with deionized water to yield an iron concentration of 10 µg/ml. Isotope-spiked standards for an instrument calibration will be prepared in the same manner as with the unknown samples. Isotope ratios for 58Fe/54Fe and 57Fe/54Fe in blood before and after concurrent enteral (58Fe) and parenteral (57Fe) administration of isotopes will be determined by a computer-controlled thermal ionization mass spectrometer using an Elan Model 250 ICP/MS system, (SCIEX, Inc., Thornhill, Ontario, Canada), operated in the isotope ratio mode, as followed by Zlotkin et al. in the Department of Pediatric and Nutritional Science, University of Tronoto, Canada.