A critical assessment of laser ablation ICP-MS as an analytical tool for depth analysis in silica-based glass samples

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a versatile technique for trace element analysis with respect to depth in solid samples. The high sensitivity of ICP-MS makes it possible to determine most elements in the periodic table at trace levels (< 1 mu g g(-1)). Recent trends in the development of instrumentation have led to the possibility of analysing craters of smaller diameter with variable depth. However, a major limitation to this approach for depth profiling, preventing accurate and precise analysis, is element-selective, non-reproducible ablation. The ability for representative sampling during depth analysis is tested in this study by ablating into homogeneous silica-based glass materials. Elemental relative response deviations of up to 300% are observed for selected elements during progressive ablation into the glass target. The geometry of the ablation crater controls the accuracy of sampling of the material at depth. Elemental fractionation becomes significant for some elements (e.g., Zn, Pb) when the depth/diameter ratio of the ablation crater is > 6, corresponding to a 50% reduction in analyte response. Large diameter craters, if ablated with sufficient laser power density, reduce elemental fractionation and give a larger signal for a longer period of time, providing more suitable conditions for representative analysis. LA-ICP-MS can be a powerful technique for depth profiling provided that optimum analytical conditions are selected.