Abstract

Inductively coupled plasma mass spectrometry (ICP-MS) is a very useful method for the determination of long-lived isotopes of uranium, thorium and other elements. This study investigates parameters for measurement of isotopes of these elements in human urine using a magnetic sector ICP-MS method and a previously established quadrupole ICP-MS method. The investigation was performed by means of comparison of data from the determination of ²³⁸U, ²³⁵U and ²³²Th in urine-based quality control materials, as well as specimens from an investigation of occupational exposure to smoke and particulates from a population of firefighters. In addition to difference in instrumentation, additional isotopes were quantitated, and sample size was decreased to 100 µL for the magnetic sector method from 500 µL for the quadrupole method. Though non-instrumental bias in samples >500 ng L⁻¹²³⁸U due to differences in the serial dilution technique (Digiflex diluter for quadrupole, Eppendorf pipettor for the magnetic sector method) was noted, results of comparison of the analyses of samples by magnetic sector ICP-MS compare very favorably with those from quadrupole ICP-MS (rsquared = 0.991 inclusive, 0.987 excluding serially diluted samples), with a near unity slope (0.934 inclusive, 1.06 excluding serially diluted samples) and near-zero intercept (0.0105 inclusive, 0.001 excluding serially diluted samples). Addition of calibration with ²³⁵U (0.72% in NIST U standard) allowed the characterization of QCs and measurement of this isotope using the magnetic sector method with a multi-run (n = 28) urine LOD of 0.06 ng L⁻¹. Other magnetic sector urinary isotopic multi-run LODs were in the below 3 ng L⁻¹ range for a 100 µL sample versus 4 ng L⁻¹ for a 500 μL sample with the quadrupole (n = 43).