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.
|Original language||English (US)|
|Number of pages||10|
|Journal||Journal of Analytical Atomic Spectrometry|
|State||Published - Jun 2020|
All Science Journal Classification (ASJC) codes
- Analytical Chemistry