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

Abstract

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 <5×10−6, 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 <5×10−6, 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.