Isotopic analysis of Fe in human red blood cells by multiple collector-ICP-mass spectrometry

Precise Fe-56/(54) Fe and Fe-57/Fe-54 isotopic ratios on human red blood cell (RBC) samples have been measured using multiple collector-ICP-mass spectrometry (MC-ICPMS). The mass spectrometric interferences on Fe isotopes (e.g., (ArO+)-Ar-56 and (ArOH+)-Ar-57) were successfully minimized by a dry plasma condition achieved by a desolvating nebulizer sample-introduction technique. In order to eliminate possible variations in the measured isotopic ratios due to non-mass spectrometric interferences, Fe was separated from remaining organic compounds and major co-existing elements using an ion chromatographic technique. The resulting precisions of the Fe-56/Fe-54 and Fe-57/Fe-54 ratio measurements were 0.12parts per thousand and 0.20parts per thousand, respectively, which were high enough to detect the isotopic variation of Fe in nature. For an inter-laboratory comparison, all of the Fe isotopic ratio data were normalized by the ratios for the IRMM-014 international isotopic standard. A series of 12 RBC samples were collected from one person through monthly-based sampling over a period of one year. These were analyzed to test possible seasonal changes in the Fe-56/Fe-54 and Fe-57/Fe-54 ratios. Moreover, in order to test possible variations in the Fe-56/Fe-54 and Fe-57/Fe-54 ratios among different people, RBC samples were collected from five volunteers (four males and one female). The Fe-56/Fe-54 and Fe-57/Fe-54 ratios for a series of 12 RBC samples collected over a one-year period show 3.06parts per thousand and 4.51parts per thousand lower than the values of IRMM-014, and no significant seasonal change could be found in the ratios. The lack in seasonal changes in the Fe isotopic ratios could be explained by a small contribution of the daily net-intake of Fe (1 - 2 mg/day) onto the total amount of Fe in the human body (2 - 4 g). The Fe-56/Fe-54 and Fe-56/Fe-54 ratios for RBC samples collected from four male samples did not vary measurably, whereas the Fe isotopic ratios for a female RBC were 0.3parts per thousand/amu heavier than the mean value of four male samples. This difference in Fe isotopes among the individuals can be the result of a difference in uptake efficiency of the Fe through a dietary process from the digestive tract. The data obtained here demonstrate that the isotopic ratios of trace metals can provide new information about metabolic efficiencies of the metallic elements.