Isotopic Analysis of Fe in Human Red Blood Cells by Multiple Collector-ICP-Mass Spectrometry

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Precise ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe 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., ⁵⁶ArO⁺ and ⁵⁷ArOH⁺) 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 ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe ratio measurements were 0.12‰ and 0.20‰, respectively, which were high enough to detect the isotopic variation of Fe in nature. For an interlaboratory 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 ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe ratios. Moreover, in order to test possible variations in the ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe ratios among different people, RBC samples were collected from five volunteers (four males and one female). The ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe ratios for a series of 12 RBC samples collected over a one-year period show 3.06‰ and 4.51‰ 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 ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe ratios for RBC samples collected from four male samples did not vary measurably, whereas the Fe isotopic ratios for a female RBC were 0.3‰/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.