Uranium–lead isotopic analysis from transient signals using high-time resolution-multiple collector-ICP-MS (HTR-MC-ICP-MS)

Abstract

²⁰⁶Pb/²³⁸U isotopic ratios were measured from transient signals produced by laser ablation for a short time period (4 shots with 60 kHz repetition rates). To investigate the effect of the dwell time on the measured ratios, the signal intensities of ²⁰⁶Pb and ²³⁸U were obtained with various dwell times (200, 20, 2, 0.2, 0.1, 0.04, and 0.02 ms). The signal intensities of ²⁰⁶Pb and ²³⁸U increase with shortening the dwell time, and the resulting peak signal intensities of ²³⁸U for three zircons (Plešovice, GJ-1, and Nancy 91500) exceeded 10⁶ cps with the shortest dwell time (0.02 ms). This is mainly due to moderation of the signal intensity of ²³⁸U through averaging the intensity data from neighboring time slices, suggesting that the acquired signal intensity profile does not reflect the actual signal intensities. Based on the moderated signal intensity data, correction of the counting loss due to the detector dead time can be erroneous, and this can be well demonstrated by the changes of the measured ²⁰⁶Pb/²³⁸U ratios with different dwell times. With the correction of the counting loss based on the obtained data and a short dwell time (0.02 ms), the resulting ²⁰⁶Pb/²³⁸U ratios for zircon and glass standard materials were in good agreement with the literature within the analytical uncertainties achieved in this study (2–3%). The data obtained here demonstrate clearly that the acquisition with a short dwell time is essential to obtain reliable elemental/isotopic data from transient signals emanating from single-shot laser ablation and introduction of nanoparticles.