A practical approach to determination of low concentration uranium isotope ratios in small volumes of urine

Abstract

Thermal ionization-mass spectrometry (TIMS) has long been the benchmark technique for determining uranium isotope ratios. Instrumentation improvements and application optimizations on inductively coupled plasma-mass spectrometry (ICP-MS) instruments have enabled acquisition of data approaching the accuracy and precision obtained using TIMS. However, problems remain for urine uranium ICP-MS isotope ratio analyses at low concentrations. Microwave digestion of urine to eliminate organic interferences, solid phase chelation extraction of actinides from the matrix, use of a desolvating introduction system, and gas flow optimizations to minimize mass fractionation were employed to develop a magnetic sector ICP-MS method sensitive enough to accurately measure urine uranium isotope ratios at concentrations below 10 ng L⁻¹ in 2 mL urine with little or no correction. This method was used to determine isotope ratios for internally and externally prepared natural and depleted uranium-spiked urine. Ratio accuracy obtained from triplicate digestions, preparations, and analyses were within 1% of expected values characterized or calculated from sources over concentrations ranging from 5 to 500 ng L⁻¹ total uranium.