In situ isotopic analysis of uranium using a new data acquisition protocol for 1013 ohm Faraday amplifiers

Abstract

Isotopic ratio measurements for ²³⁵U/²³⁸U and ²³⁴U/²³⁸U ratios were made for a series of solid materials using laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). To extend the dynamic range of the isotopic ratio measurements, three Faraday detectors with different amplification gains (amplifiers with 10¹¹ and 10¹³ ohm resistors) were employed. To reduce the sampling volume, isotopic ratio measurements were made on transient signals obtained by short-time LA duration (<3 s). From the transient signals obtained using the LA technique, measured ²³⁵U/²³⁸U ratios were approximately 3% higher than values noted in the literature when a conventional tau correction technique was employed. This was mainly due to incomplete correction of the slow response of the Faraday amplifiers. To overcome this, continuous ion-monitoring, rather than tau correction, was applied to the isotopic ratio measurements from the transient signals. Typical precision for the ²³⁵U/²³⁸U and ²³⁴U/²³⁸U ratio measurements were 1‰ and 7% (2SD), respectively, and the resulting ²³⁵U/²³⁸U ratios for zircons and glass materials showed good agreement with values observed in the literature. The isotopic ratio measurements revealed heterogeneity of ²³⁵U/²³⁸U ratios among titanite grains. The measured ²³⁴U/²³⁸U ratios were consistent with the value of the secular equilibrium in the ²³⁸U series, except for FCT titanite. Herein, a possible cause for the discrepancy in the measured ²³⁴U/²³⁸U ratio in the secular equilibrium is discussed. The data obtained in this study demonstrate that the present data acquisition protocol can serve as a benchmark technique for isotopic ratio measurements derived from transient signals using Faraday detectors, particularly when using high-gain (e.g. 10¹³ ohm resistors) amplifiers.