A novel method for measuring ultra-trace levels of U and Th in Au, Pt, Ir, and W matrices using ICP-QQQ-MS employing an O2 reaction gas

Abstract

Increased demand for improving ultra-low background detection capabilities for rare-event fundamental physics applications has resulted in the need for fast, convenient and clean assay methodologies that either preclude or reduce chemical sample pre-processing. A novel method for the measurement of ultra-trace concentrations (fg g⁻¹ level) of natural ²³²Th and ²³⁸U and non-natural tracer isotopes ²²⁹Th and ²³³U was demonstrated in a solution of 10 μg g⁻¹ each of Au, Pt, Ir, and W in 2% HNO₃ using an ICP-QQQ-MS. Polyatomic interference across an m/z range of 227–239 was characterized: the major interferent with ²²⁹Th⁺ is ¹⁹⁴Pt³⁵Cl⁺; interferents with ²³²Th⁺ are ¹⁸⁴W¹⁶O₃⁺, ¹⁸³W¹⁶O₃H⁺, ¹⁹²Pt⁴⁰Ar⁺, ¹⁹⁶Pt³⁶Ar⁺, ¹⁹⁵Pt³⁷Cl⁺, and ¹⁹⁷Au³⁵Cl⁺; those with ²³³U⁺ are ¹⁹³Ir⁴⁰Ar⁺, ¹⁹⁷Au³⁶Ar⁺, and ¹⁸⁴W¹⁶O₃H⁺; and that with ²³⁸U⁺ is ¹⁹⁸Pt⁴⁰Ar⁺. Scanning the selected m/z range of 227–270 showed that higher oxide polyatomic species from the matrix elements either did not form or did not create a significant background on the target analyte masses. All measured concentrations in standard solutions matched the target values within the 98% confidence interval. The Th measurements were 80% accurate or better at the 10 fg g⁻¹ level and above, and the U measurements were 90% accurate or better at the 10 fg g⁻¹ level and above. Measurements at the 1 fg g⁻¹ level were consistent with target values within 1 standard deviation, although the standard deviations of all three replicates were greater than 20% of the measured concentration value. Method detection limits in the matrix solutions were calculated to be 2.74 fg Th and 12.9 fg U. In an electronic sample, which typically has 0.1% precious metal content, our method would give detection limits of 274 fg Th and 1291 fg U given a maximum of 10 μg g⁻¹ coinage metal matrix. This method is but one example of how state-of-the-art quadrupole mass spectrometry and collision reaction cell technology can be leveraged to develop novel analytical capability at ultra-trace levels.