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_2 reaction gas

摘要

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 ng 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_3H~+, ~(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_3H~+; 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.