Optimisation of laser and mass spectrometer parameters for the in situ analysis of Rb/Sr ratios by LA-ICP-MS/MS

Abstract

The Rb–Sr isotopic system is widely used in geochronology. Conventionally burdened by the isobaric overlap of ⁸⁷Rb and ⁸⁷Sr, Rb/Sr dating in situ has only recently become achievable with the newly developed LA-ICP-MS/MS system. Simultaneous use of reactive gas (e.g. O₂, N₂O, or CH₃F) during LA-ICP-MS/MS analysis has been shown to resolve the Rb and Sr overlap, thus now making available key spatial and temporal information that can only be accessed via in situ analytical techniques. The accuracy and precision of Rb/Sr ratios and ages are largely dependent on the laser and ICP-MS/MS parameters used. Rb/Sr isotopic analysis by LA-ICP-MS/MS is a recently developed technique and these parameters are yet to be fully explored. We investigate the effects of laser wavelength (213 nm and 193 nm), laser frequency (5 Hz and 10 Hz), laser carrier gas (He, H₂, and N₂), dwell time, and external standard calibration on the accuracy and precision of ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr ratios and ages. These analytical conditions have been tested on the commercially available reference materials: NIST SRM 610, USGS BHVO-2G, and pressed nano-particulate powder tablet CRPG Mica-mg, as well as a Monastery phlogopite megacryst. Our results show that accuracy and precision for ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr ratios are significantly affected by laser wavelength and frequency. Variation in these parameters can strongly magnify any matrix effects which directly influences the ability to apply effective external corrections. We obtain the best accuracy and precision when using a 193 nm laser wavelength, ablating at a frequency of 5 Hz (0.30 2s% and 0.15 2s% for ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr ratios, respectively). Meanwhile we find that age accuracy is highly dependant on external reference materials. When these analytical settings are put to test on the Monastery phlogopite, we obtain an age of 90.0 ± 3.6 (0.24% accuracy) when using mica-mg (⁸⁷Rb/⁸⁶Sr) and NIST 610 (⁸⁷Sr/⁸⁶Sr) as external standards.