PNNL

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 technique for the measurement of ultra-trace concentrations (fg·g-1 level) of natural 232Th and 238U and non-natural tracer isotopes 229Th and 233U were measured in a solution of 10 µg·g-1 each of Au, Pt, Ir, and W in 2% HNO3 using an ICP-QQQ-MS. Polyatomic interferences across a m/z range of 227-239 were characterized: the major interferents with 229Th+ is 194Pt35Cl+; with 232Th+ are 184W16O3+, 183W16O3H+, 192Pt40Ar+, 196Pt36Ar+, 195Pt37Cl+, and 197Au35Cl+; with 233U+ are 193Ir40Ar+, 197Au36Ar+, 184W16O3H+; and 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 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 have 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.