Development and application of LA-ICP-MS in the geosciences: past, present and future

The rapid advances in in situ laser ablation (LA) inductively coupled plasma-mass spectrometry (ICP-MS) and multicollector (MC)-ICPMS have provided datasets in geochronology and geochemistry that have revolutionised our understanding of the geodynamic Earth at all scales. The development and application of LA-ICP-MS continues to grow at a dramatic rate and in situ analyses for elements and isotopic ratios are now performed routinely in numerous laboratories worldwide. Like other microbeam techniques LA-ICP-MS provides the benefit of high spatial resolution and produces data that can be interpreted in a microstructural context. The development of new LA-ICP-MS methodologies has been enabled by advances in instrumentation in conjunction with studies of the fundamental processes involved in ablation and in the ICP (e.g. laser-induced elemental and isotopic fractionation, plasma loading, mass bias). The emergence of the multi-collector ICP-MS for high-precision in situ measurement of radiogenic (e.g. Hf in zircon) and 'non-traditional' stable isotopes (e.g. Cu and Fe in sulfides) has revolutionised analytical geochemistry. On the laser front, Nd:YAG (266, 213 or 193 nm) or ArF excimer (193 nm) remain the most commonly used laser sources. In comparison with these nanosecond pulse-width systems, ablation using femtosecond lasers has been shown to approach stoichiometric sampling and reduce laser-induced fractionation effects. However the high cost of the commercial femtosecond systems has restricted their uptake. Despite the significant advances of the last decade, the continued rapid spread and acceptance of the technology may be jeopardized while any significant analytical issues remain unsolved. The accuracy and precision of in situ isotope ratio measurements are inherently lower than solution measurements because of the complexity of matrix effects and corrections for mass bias and isobaric interferences. Optimising precision of elemental and isotope ratio measurements while maintaining spatial resolution brings challenges, and emphasises the need for improved understanding of measurement uncertainties and error budgets. Well characterized and readily available reference materials combined with more inter-laboratory comparison exercises are essential. Future goals for LA-ICP-MS include a quantum increase in sensitivity, fine-scale compositional mapping of geological samples, and overcoming effects of elemental and isotopic fractionation. Elimination of elemental fractionation will accelerate development of procedures to measure major, minor and trace-element abundances without depending on matrix-matched calibration materials and independently determined internal standard concentrations.