Microanalysis of Pb isotope ratios of low-Pb glass samples by femtosecond laser ablation-multiple ion counter-ICP-mass spectrometry (fsLA-MIC-ICP-MS)

摘要

We report high-resolution in situ Pb isotope analysis (207Pb/206Pb and 208Pb/206Pb ratios) by femtosecond laser ablation-multiple ion counter-inductively coupled plasma-mass spectrometry (fsLA-MIC-ICP-MS) for low-Pb geological reference glasses. Pb isotope analysis was realized using a multiple ion counter (MIC) and a modified inductively coupled plasma (ICP) ion interface to improve the signal-to-noise ratio and instrumental sensitivity, respectively. Use of femtosecond deep-UV (200 nm) laser ablation enhanced the sampling efficiency of the small amount of glass. Pb memory from sample-skimmer cones were subtracted using an on-peak background method. Instrumental mass bias correction and inter-MIC calibration were performed simultaneously using a standard-sample bracketing method. The optimized analytical protocol was applied to various rock reference glasses (BHVO-2G, BCR-2G and GSD-1G from the United States Geological Survey, and the MPI-DING series glasses: KL2-G, StHs6/80-G, ATHO-G, T1-G, and GOR132-G from the Max Plank Institute for Chemistry, Germany) with spatial resolution of ~30-μm diameter and 3-25-μm depth. The accuracy achieved was better than 0.38% compared with reference values, and the reproducibility was better than 1.0% (2SD) for both 207Pb/206Pb and 208Pb/206Pb ratios from the glasses with Pb concentrations of 1.7-19 ppm. We found that proper control of signal intensity is crucial for accurate and precise isotopic ratio measurements by the miniature MIC. Signal intensity higher than 300 kcps results in instantaneous saturation of the MIC and consequent inaccurate isotopic ratio measurements. The optimization of the fsLA-MIC-ICP-MS system allows high-throughput Pb isotopic microanalysis with the precision, accuracy and lateral spatial resolution comparable to those of secondary ion mass spectrometry, indicating the versatility of this method for in-situ microanalysis of Pb isotopes in the geosciences.