Evaluation of infrared femtosecond laser ablation for the analysis of geomaterials by ICP-MS

The capabilities of an infrared (IR) Ti:sapphire femtosecond laser (approximate to 800 nm) to ablate and analyze geomaterials such as monazite, zircon and synthetic glass reference materials is evaluated, with emphasis on U/Pb ratio determinations useful for dating accessory minerals in rocks. We particularly discuss the influence of pulse duration ( respectively 60, 200, 350, 500, 670, 830, 2000 and 3000 fs) on the internal precision ( 2 min ablation), reproducibility over two weeks and accuracy of quadrupole ICP-MS measurements. The best results for all these criteria are obtained when using the shortest pulse duration ( 60 fs). It was found that internal precision and reproducibility were improved by a factor of 3 and 4, respectively, from picosecond to 60 fs pulsewidths. Reproducibility at this pulse duration for U/Pb ratio determinations is of 2% RSD or better, depending on the material analyzed, and this ratio is accurate within this uncertainty. Lead isotopic ratios also benefit from the shortest pulsewidth. They are measured at 60 fs with a precision (< 0.5% RSD) approaching the limitations of quadrupole ICP-MS. Preliminary data were also obtained using the 3rd harmonic (approximate to 266 nm) of the Ti: sapphire fundamental wavelength and they are compared with the infrared mode. There seems to be no obvious analytical benefit to switch from IR to UV in the femtosecond laser ablation regime. Analyses of zircon 91500 with IR pulses led to better repeatability, around 0.9% ( 10 values, 1 sigma), compared to 3% for the UV pulses. The accuracy appears to be comparable for the two wavelengths.