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 O reaction gas

• Published in: Journal of analytical atomic spectrometry Vol. 35; no. 12; pp. 2859 - 2866
• Main Authors: Harouaka, Khadouja, Hoppe, Eric W, Arnquist, Isaac J
• Format: Journal Article
• Published: 08.12.2020
• ISSN: 0267-9477; 1364-5544
• DOI: 10.1039/d0ja00220h

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 232 Th and 238 U and non-natural tracer isotopes 229 Th and 233 U was demonstrated in a solution of 10 μg g −1 each of Au, Pt, Ir, and W in 2% HNO 3 using an ICP-QQQ-MS. Polyatomic interference across an m / z range of 227-239 was characterized: the major interferent with 229 Th + is 194 Pt 35 Cl + ; interferents with 232 Th + are 184 W 16 O 3 + , 183 W 16 O 3 H + , 192 Pt 40 Ar + , 196 Pt 36 Ar + , 195 Pt 37 Cl + , and 197 Au 35 Cl + ; those with 233 U + are 193 Ir 40 Ar + , 197 Au 36 Ar + , and 184 W 16 O 3 H + ; and that with 238 U + is 198 Pt 40 Ar + . 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. A mass shift method using ICP-MS/MS with O 2 reaction gas is used for ultra-trace determinations of U and Th in samples containing W, Ir, Pt, and Au derived polyatomic interferants without the need for extensive chemical sample preprocessing.