Determination of Picogram-per-Gram Concentrations of ²³¹Pa and ²³⁰Th in Sediments by Melt Quenching and Laser Ablation Mass Spectrometry

Abstract

Uranium, thorium, and protactinium radionuclides in marine sediments are important proxies for understanding the earth’s environmental evolution. Conventional solution-based methods, which typically involve isotope spike preparation, concentrated acid sample digestion, column chemistry, and mass spectrometry, allow precise but time-consuming and costly measurements of these nuclide concentrations (i.e., 230Th and 231Pa). In this work, we have established an efficient method for 230Th and 231Pa measurement of marine sediments down to the picogram-per-gram level without purification and enrichment. Our method first transforms a small amount of thermally decomposed sediments (∼0.1–0.2 g) to homogeneous silicate glass using a melt quenching technique and then analyzes the glass with laser ablation multicollector inductively coupled plasma–mass spectrometry. Standard sample bracketing with isotope-spike-calibrated glass standards prepared in this study was used to correct for instrumental fractionation during measurement. It is demonstrated that our method can accurately determine the U–Th–Pa concentrations of typical marine sediments in the late Pleistocene with precision of a few percent. Compared with the conventional solution-based methods, the turnover time of sample preparation and measurement with our established protocol is greatly reduced, facilitating future application of U-series radionuclides in reconstructing oceanic processes at high temporal and spatial resolution.

Ice sheet and precession controlled subarctic Pacific productivity and upwelling over the last 550,000 years

“…Sample preparation and analysis followed the methods described in ref. 58. Briefly, about 0.2-0.3 g of dried sample was homogenized, heated to remove volatile components, packed in a molybdenum capsule and isolated in a graphite tube to avoid oxidation, then heated at 1300 °C for 10-15 min before being quenched in water.…”

“…Briefly, about 0.2-0.3 g of dried sample was homogenized, heated to remove volatile components, packed in a molybdenum capsule and isolated in a graphite tube to avoid oxidation, then heated at 1300 °C for 10-15 min before being quenched in water. By doing this, the samples were transformed into homogeneous glass 58 . Analysis of 230 Th, 232 Th, and 238 U in glass samples were done by the standard sample bracketing method, on a Neptune plus MC-ICP-MS, coupled with a New Wave 193 Research Laser Ablation system in the State Key Laboratory for Mineral Deposits Research in Nanjing University.…”