Development and application of a Raster-spot model in Galvo-fsLA-ICP-MS for precise trace-element and U-Pb isotopic analysis

Abstract

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with femtosecond (fs) laser sources offers significant advantages in reducing elemental fractionation and matrix effects compared to nanosecond systems. However, conventional galvanometer-scanned femtosecond laser ablation (Galvo-fsLA) systems suffer from rapid signal decay (90%-97% within 20 seconds), limiting their application for analyses requiring sustained stable signal output. Here we introduce a novel Raster-spot model for Galvo-fsLA systems that fundamentally restructures the ablation sequence by decomposing individual scanning layers into discrete sub-cycles. This approach maintains stoichiometric ablation while drastically reducing signal decay. Scanning electron microscopy reveals that the Raster-spot model produces craters with notably flatter bases. Quantitative trace element analysis of 42 elements in five reference materials OA-1, OH-1, OJY-1, BHVO-2G, NKT-1G demonstrates relative errors and relative standard deviations predominantly below 15%. For U-Pb geochronology, the Raster-spot model achieves exceptional accuracy with relative errors almost below 0.5% for zircon standards (Tanz, SA01, Plešovice) and other accessory minerals (apatite, monazite, rutile, allanite, titanite). Optimal optical focusing is also critical, with deviations as small as ± 20 μm degrading accuracy from -0.2% to -1.3%. The Raster-spot model represents a significant advancement in Galvo-fsLA-ICP-MS methodology, enabling high-sensitivity, long-duration analyses with superior accuracy for both trace element quantification and U-Pb dating across diverse geological materials.