William R Keyes

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 µg L−1versus the informational value of 1.07 ± 0.04 µg L−1. Total Mo and 97Mo were measured in plasma collected from a human subject following intravenous adminstration of 33 µg 97Mo, 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.

Isotope ratios of calcium determined in calcium-46 enriched samples from infants by automated multiple-collector thermal ionization mass spectrometry

A high-precision method was developed for automated multiple-collector determination of 46Ca enrichment using magnetic sector, thermal ionization mass spectrometry (TIMS). Calcium was separated from biological samples by precipitation as calcium oxalate. The calcium oxalate was washed, then dissolved in nitric acid, and isotope ratios were determined in 10 µg samples. The 44Ca, 46Ca and 48Ca isotopes were collected simultaneously. The 46Ca:48Ca ratios were measured and corrected for fractionation by iterative normalization, using the 44Ca:48Ca ratio, in order to achieve the required precision. Blocks of ten ratios were measured with an internal or within-run precision of 0.14% relative standard deviation (RSD) in urine samples and 0.10% RSD in faecal samples. The external or between-run precision for nine replicates was 0.07% RSD for urine and 0.09% RSD for faecal samples. Enrichment of samples collected following the feeding of 46Ca to pre-term infants ranged from 8 to 179 Δ% excess in faecal samples and from 19 to 91 Δ% excess in urine samples. If another isotope in addition to 46Ca is to be enriched, the identical analytical method can be applied by collecting 42Ca, 43Ca, 44Ca and 46Ca simultaneously, using the two unenriched isotopes to correct for fractionation.