Optimizing MC-ICP-MS with SEM protocols for determination of U and Th isotope ratios and 230Th ages in carbonates
Date of Issue2019
Asian School of the Environment
Earth Observatory of Singapore
We have optimized techniques to measure uranium and thorium isotopic compositions using secondary electron multiplier (SEM) and peak-jumping protocols on a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS, Thermo Scientific Neptune Plus). This instrument is equipped with an advanced interface pump, X- skimmer cone, Jet sample cone, and desolvating nebulizer system (CETAC Aridus II) that enables overall ioni-zation and transmission efficiency up to 3% for both U and Th. Approximately 50 fg234U or 230Th can routinely yield a precision of 1–2‰ (2σ) for abundance and isotopic measurements. Non-linear behavior was not observed in the MasCom SEM with a retarding potential quadrupole in our MC-ICP-MS system when the beam intensity was limited to < ∼1 × 10 6 cps. We performed a series of tests to examine the influence of tuning parameters, peak shift, and operating voltage of the SEM on isotopic measurements. Results for U/Th standards are consistent with published values and demonstrate the reliability of our protocols. Lastly, the precision and accuracy of 230Th ages based on U and Th isotope ratios is presented for a variety of carbonates, including both modern and secular-equilibrium samples. For corals as young as ∼10-yr old, we have significantly reduced the procedural chemistry blanks so that only 40 mg is required to yield an analytical precision of ± 1.4 yr (2σ). All 230Th ages from modern and fossil corals and speleothems are in agreement with band counting ages.
© 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).