In situ Analysis of Mg Isotopic Compositions of Basalt Glasses by Femtosecond Laser Ablation Multi-collector Inductively Coupled Plasma Mass Spectrometry

A method was developed for the in situ analysis of magnesium isotopes by femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry (fsLA-MC-ICP-MS). Concentration matching experiment revealed that the Mg isotopic composition could be measured accurately when Mg concentration difference between sample and standard were in the range of 0.4-3.0. The Mg isotopic composition was affected by the femtosecond laser conditions, including laser spot size and scanning speed, due to mass discrimination changing which was caused by different mass loading into mass spectrometer. The amount of ablated aerosol was correlated with laser frequency, laser ablation spot size, and scanning speed. However, the laser energy density showed negligible effect on the accuracy of Mg isotope measurement due to the high energy output of femtosecond laser which was higher than the ablation threshold. To obtain reliable Mg isotopic compositions, the laser ablation spot size, laser frequency and scanning speed should be consistent during one analytical section and the difference of Mg concentration between standard and samples should be in 3 times. The results of two reference materials agreed well with the published values within 2 s analytical uncertainties. The developed method is simple, reliable and accurate, and can be used for the Mg isotope analysis of volcanic glasses and silicate minerals.