Flow-injection technique for determination of uranium and thorium isotopes in urine by inductively coupled plasma mass spectrometry

A sensitive and efficient flow-injection (FI) preconcentration and matrix-separation technique coupled to sector field ICP–mass spectrometry (SF-ICP–MS) has been developed and validated for simultaneous determination of ultra-low levels of uranium (U) and thorium (Th) in human urine. The method is based on selective retention of U and Th from a urine matrix, after microwave digestion, on an extraction chromatographic TRU resin, as an alternative to U/TEVA resin, and their subsequent elution with ammonium oxalate. Using a 10 mL sample, the limits of detection achieved for ²³⁸U and ²³²Th were 0.02 and 0.03 ng L^–1, respectively. The accuracy of the method was checked by spike-recovery measurements. Levels of U and Th in human urine were found to be in the ranges 1.86–5.50 and 0.176–2.35 ng L^–1, respectively, well in agreement with levels considered normal for non-occupationally exposed persons. The precision obtained for five replicate measurements of a urine sample was 2 and 3% for U and Th, respectively. The method also enables on-line measurements of the ²³⁵U/²³⁸U isotope ratios in urine. Precision of 0.82–1.04% (RSD) was obtained for ²³⁵U/²³⁸U at low ng L^–1 levels, using the FI transient signal approach.