Radiochemical neutron activation analysis of zinc isotopes in human blood, urine, and feces for in vivo tracer experiments

Enriched stable isotopes are being increasingly used for study of trace element nutrition in humans who cannot be studied by use of in vivo radioactive tracers (e.g., subjects under age 18 and pregnant women). Zinc metabolism in these subjects can be evaluated by administration of Zn enriched to 65% in the minor isotope, 70Zn (0.6% natural abundance). The enhanced 70Zn is detected later in red blood cells, plasma, urine, and feces by measuring 70Zn/64Zn or 70Zn/68Zn ratios. Stable isotope concentrations are measured by neutron activation of the samples and observation of their products: 244-day 65Zn, 14-h 69mZn, and 4-h 71mZn. Zinc-65 can be observed in these samples without chemical separations 3 weeks after irradiations, but large amounts of 24Na and other short-lived species preclude direct observation of the short-lived Zn activities. Preirradiation chemistry was developed to remove most interferences, the major steps being to place the sample on Chelex resin, elute alkali metals and alkaline earths from it, and irradiate the resin containing the Zn. gamma-Rays of 69mZn can be observed on the irradiated resin, but additional precipitation and solvent extraction steps are needed to remove 56Mn and 64Cu for clear observation of 71mZn and 65Zn within hours after irradiation. Yields for pre- and postirradiation separations are typically 85% and 70%, respectively. The stable isotope tracer method was validated by simultaneous in vivo tracing with radioactive 65Zn in four subjects.