Hemin and plasma protein isolated from normal blood were found to have a slightly increased abundance of N15 over tank nitrogen or ammonium chloride. The mean values for normal hemin, 0.0046, and for normal plasma protein, 0.0039 atom % excess N15, were similar to the N15 concentration found in urinary urea. These results lend further support to the idea that minute fractionation of the nitrogen isotopes might occur through normal biological processes.
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Abbott et al. (1953) studied this question.