Measurements of natural uranium concentration and isotopic composition with permil-level precision by inductively coupled plasma-quadrupole mass spectrometry

A new analytical technique using inductively coupled plasma-quadrupole mass spectrometry (ICPQMS) has been developed that produces permil-level precision in the measurement of uranium concentration ([U]) and isotopic composition (δ²³⁴U) in natural materials. A ²³³U-²³⁶U double spike method was used to correct for mass fractionation during analysis. To correct for ratio drifting, samples were bracketed by uranium standard measurements. A sensitivity of 6-7 × 10⁸ cps/ppm was generated with a sample solution uptake rate of 30 mL/min. With a measurement time of 15-20 min, standards of 30-ng uranium produced a within-run precision better than 3%(±2 R.S.D.) for δ²³⁴U and better than 2%for [U]. Replicate measurements made on standards show that a between-run reproducibility of 3.5%for δ²³⁴U and 2% for [U] can be achieved. ICP-QMS data of δ²³⁴U and [U] in seawater, coral, and speleothem materials are consistent with the data measured by other ICP-MS and TIMS techniques. Advantages of the ICP-QMS method include low cost, easy maintenance, simple instrumental operation, and few sample preparation steps. Sample size requirements are small, such as 10-14 mg of coral material. The results demonstrate that this technique can be applied to natural samples with various matrices.