Impact of ablation cell design in LA-ICP-MS quantification

Abstract

Bulk analysis in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has commonly made use of large one-volume ablation cells of various geometries. These ablation cells offer significant dispersion of aerosols, resulting in stable signals but a loss of spatial and temporal resolution. In recent years, low dispersion ablation cells, most commonly two-volume cells, have been invented for the purposes of depth profiling and imaging due to their capabilities to resolve individual laser pulses. In this study, the differences between the two types of ablation cells were studied based on ICP-MS analysis, optical particle sizing and computational fluid dynamics. Differences in particle size distributions (PSD) and elemental ratios were investigated with respect to positional dependence within an ablation cell. 3D-gas flow patterns by CFD formed in a plain large one-volume cylindrical cell (30 cm³) revealed inhomogeneous gas flow patterns, where the location of ablation has an impact on the aerosol residence time within the cell. As a consequence, the mean particle size that is transported to the ICP varies from 80 nm to 150 nm for a cylindrical ablation cell. Correlating with the PSD, the ratio of ²³⁸U/²³²Th and ²³⁸U/²⁰⁸Pb varied by up to 15% using the same parameters but different ablation locations within the cell. As this deviation directly affects quantification, methods for matching in provenance studies, geochronology and forensics are especially vulnerable to these errors. For the low dispersion cell experiments, the recently described parallel flow ablation cell (PFAC) was used as an exemplary model. Mean particle sizes were significantly lower by a factor of more than 2, and the variation of the elemental ratios depending on the location within the cell was significantly reduced from 15% to 5%. Through use of longer transport tubing, it is possible to change from resolved laser pulses to continuous stable signal akin to large volume ablation cells, while still using the PFAC. This smoothing of the aerosol showed no effect on element ratios, resulting in comparable results as the resolved measurements. This work indicated that low dispersion ablation is more successful in achieving reproducible element ratios. It was also found that a decrease in aerosol dispersion within the ablation cell helps to suppress plasma induced fractionation effects originating from incomplete ionization.