In situ precise determination of stable Mo isotope ratios in molybdenite by femtosecond LA-MC-ICP-MS

Abstract

This study developed a new method for in situ measurement of stable Mo isotope ratio using femtosecond laser ablation connected to a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). We assessed how laser fluence, laser repetition rate, and signal intensity impact Mo isotope mass fractionation, using synthetic molybdenite Mo–H8 as the reference material. Findings reveal that the laser repetition rate has negligible impact on Mo isotope fractionation, while signal intensity and laser fluence slightly influence the precision of δ⁹⁸Mo. Significant signal intensity disparities between samples and reference materials can markedly affect the results of Mo isotopes. We analyzed natural molybdenite samples from various deposits in China with this new method, with the optimal laser parameters including a repetition rate of 10 Hz, a spot size of 20 μm, and a fluence of 1 J cm⁻². The δ⁹⁸Mo_{NIST SRM 3134} values for nine natural samples (GD-1, GD-2, YN-1, YN-2, YN-3, SX-1, SX-2, SX-3, and GX-1) align with those obtained by solution nebulization MC-ICP-MS within analytical uncertainty, demonstrating the accuracy of this new in situ Mo isotope analytical method. This new technique offers a reliable alternative to solution nebulization MC-ICP-MS in bulk analysis, enabling precise detection of Mo isotope ratio variations at a finer scale, which provides valuable insights into mineralization processes.