Determination of the isotopic composition of uranium in urine by inductively coupled plasma mass spectrometry

A simple method based on inductively coupled plasma mass spectrometry (ICP-MS) was developed to identify exposure to depleted uranium by measuring the isotopic composition of uranium in urine. Exposure to depleted uranium results in a decreased percentage of ²³⁵U in urine samples causing measurements to vary between natural uranium's 0.72% and depleted uranium's 0.2%. Urine samples from a non-depleted uranium exposed group and a suspected depleted uranium exposed group were processed and analyzed by ICP-MS to determine whether depleted uranium was present in the urine. Sample preparation involved dry-ashing the urine at 450°C followed by wet-ashing with a series of additions of concentrated nitric acid and 30% hydrogen peroxide. The ash from the urine was dissolved in 1 M nitric acid, and the intensity of ²³⁵U and ²³⁸U ions were measured by ICP-MS. After the samples were ashed, the ICP-MS measurements required less than 5 min. The ²³⁵U percentage in individuals from the depleted uranium exposed group with urine uranium concentrations greater than 150 ng L^-1 was between 0.20%0.33%, correctly identifying depleted uranium exposure. Samples from the non-depleted uranium exposed individuals had urine uranium concentration less than 50 ng L^-1 and ²³⁵U percentages consistent with natural uranium (0.7%-1.0%). A minimum concentration of 14 ng L^-1 uranium was required to obtain sufficient ²³⁵U to allow calculating a valid isotopic ratio. Therefore, the percent ²³⁵U in urine samples measured by this method can be used to identify low-level exposure to depleted uranium.