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

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 sample size, 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.