Influence of laser parameters on isotope fractionation and optimisation of lithium and boron isotope ratio measurements using laser ablation-multiple Faraday collector-inductively coupled plasma mass spectrometry

We report the origin of isotope fractionation induced by the choice of laser parameters and a method for accurate in situ determination of lithium (delta Li-7) and boron (delta B-11) isotope ratios in glasses and minerals using laser ablation multiple Faraday collector inductively coupled plasma mass spectrometry (LA-MFC-ICPMS). Laser ablation parameters were examined using 266 nm femtosecond (266FsLA) and 193 nm nanosecond excimer (193ExLA) laser ablation systems for crater diameters of 30-200 mu m. We found that higher laser repetition rates and larger crater diameters have led to enhanced fractionation of lighter isotopes, as much as -8 parts per thousand for both delta Li-7 and delta B-11. Fractionation was primarily affected by the ICP aerosol loading and secondly by the thermal fractionation at the LA site. The former was accounted for by mass loading effects, which lowered the plasma temperature and led to insufficient aerosol vaporisation. The latter was related to the molten layer on the crater walls, which resulted in coarser and heavier delta Li-7 and delta B-11 aerosols that did not reach the ICP. Both processes can result in Rayleigh fractionation during aerosol formation and vaporisation. Controlled ablation using a constant crater size, repetition rate, and high laser fluence of 193ExLA enabled reproducible ablation for the standard NIST SRM 61X glasses and unknown basalt glasses. Based on the principles of isotopic fractionation deduced from our experiments, we propose a novel ablation volume correction (AVC) protocol for accurate isotopic analyses of various samples with different matrices. Both the repeatability and the laboratory bias of the delta Li-7 and delta B-11 measurements using the new AVC protocol were better than 1 parts per thousand for samples containing a few tens to a few tens of thousands ppm Li and B. We also report significant local heterogeneity of up to several parts per thousand found in some basalt glasses, but not in NIST SRM 612 and 610.