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Issue 6, 2020
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U–Pb ID-TIMS geochronology using ATONA amplifiers

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We document the performance of new ATONA (‘aA to nA’) amplifiers installed on an Isotopx Phoenix thermal ionisation mass spectrometer (TIMS) at Princeton University and evaluate their suitability for high-precision analyses of Pb and U isotopes in pg- to ng-size samples characteristic for U–Pb geochronology. The new amplifiers are characterised by low and stable noise levels comparable to 1012 to 1013 ohm resistors, response time <0.5 s, exceptional gain stability <1 ppm and a vast dynamic range theoretically allowing to quantify signals from aA (10−18 A) to nA (10−9 A) level. We measured a set of Pb standards, synthetic U–Pb solutions and natural zircons at currents of 2 × 10−16 to 2 × 10−12 A (corresponding to intensities of 20 μV to 200 mV relative to a 1011 ohm amplifier) to assess the utility of ATONA in replacing ion counting for the smallest samples. The results show a clear precision benefit of using ATONA-Faraday detection over Daly ion counting for ion currents of >10−14 A (1 mV relative to a 1011 ohm amplifier or ca. 60 kcps). As such currents are routinely achievable for major Pb peaks of interest (205–208Pb) in natural samples containing more than ca. 10 pg Pb* (radiogenic Pb), we expect ATONA-Faraday detection to find broad applications in U–Pb geochronology. Its practical use for low-blank, radiogenic samples continues to require ion counting for 204Pb, either with a fixed Faraday–ion counter gain or using a dynamic two-step (e.g. FaraDaly) method. Routine adoption of ATONA-Faraday collection in place of ion counting for most major Pb and U isotopes has the potential to increase sample throughput and precision, both improving the accessibility of isotope dilution (ID)-TIMS geochronology and pushing this technique towards better reproducibility.

Graphical abstract: U–Pb ID-TIMS geochronology using ATONA amplifiers

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Supplementary files

Article information

31 Mar 2020
21 May 2020
First published
22 May 2020

J. Anal. At. Spectrom., 2020,35, 1207-1216
Article type

U–Pb ID-TIMS geochronology using ATONA amplifiers

D. Szymanowski and B. Schoene, J. Anal. At. Spectrom., 2020, 35, 1207
DOI: 10.1039/D0JA00135J

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