Determination of 236U/ 238U and 235U/ 238U isotope ratios in human urine by inductively coupled plasma mass spectrometry

Isotopic ratios of urinary uranium (U) provide a decisive tool in diagnosing a patient's potential exposure to depleted uranium (DU). This study investigated the measurement of urinary ²³⁶U/ ²³⁸U and ²³⁵U/ ²³⁸U ratios of ultra trace concentrations using a sector field inductively coupled plasma mass spectrometer (ICP-MS). Uranium was separated from the urinary matrix and pre-concentrated by co-precipitation with calcium and magnesium after addition of ammonium hydroxide. The precipitates were collected, dissolved and analyzed by ICP-MS. ²³⁵U/ ²³⁸U and ²³⁶U/ ²³⁸U ratios were measured using a desolvating sample introduction system and a sector field ICP-MS. These ratios were accurately measured in samples containing as low as 5ngL ^-1 U. ²³⁶U measurement was hindered by abundance sensitivity limitations and uranium hydride ( ²³⁵UH) formation. Measured ²³⁵U/ ²³⁸U and ²³⁶U/ ²³⁸U ratios were about 7×10 ^-3 and -6, respectively, in urine samples from patients not exposed to DU. The ²³⁵U/ ²³⁸U ratio was consistently about 2×10 ^-3 in 12 urine samples from patients with embedded fragments of DU, while the corresponding ratio of ²³⁶U/ ²³⁸U ranged from 9×10 ^-6 to 33×10 ^-6. This wide range in ²³⁶U/ ²³⁸U ratios might suggest different sources of depleted uranium in those DU-urine samples. It is proposed that ²³⁵U/ ²³⁸U and ²³⁶U/ ²³⁸U ratios are 7×10 ^-3 and -6, respectively, in urine samples from individuals not exposed to DU, and 2×10 ^-3 and >7×10 ^-6, respectively, in urine samples from DU-exposed individuals.