Determination of molybdenum and enriched Mo stable isotope concentrations in human blood plasma by isotope dilution ICP-MS

An isotope dilution method was developed to measure total molybdenum and molybdenum stable isotope concentrations in 0.5 mL of human blood plasma by ICP-MS. To minimize contamination, a microwave digestion method was developed without molybdenum separation. Plasma aliquots were weighed into PTFE beakers placed on supporting PTFE beakers inside PFA liners, adding 1.4 mL HNO3 to the plasma and 8 mL HNO3 to the liner base. The samples were digested in five 10-min steps under pressure control from 20 to 100 psi. Repeated microwave digestions of the liners and beakers reduced processing blanks from 6.6 to 0.03 ng Mo. The samples were analyzed by ICP-MS. Analysis of a reference human serum produced an average molybdenum concentration of 1.02+/-0.12 mg L-1 versus the informational value of 1.07+/-0.04 mg L-1. Total Mo and Mo-97 were measured in plasma collected from a human subject following intravenous adminstration of 33 mug Mo-97, with concentration RSDs <2% for ten replicates. This ICP-MS isotope dilution method with microwave digestion can be used to determine the total molybdenum and molybdenum stable isotope concentrations in as little as 0.5 mL of human blood plasma, allowing the measurement of the appearance of molybdenum stable isotope tracers in the blood.