Chemical fractionation during infrared and ultraviolet laser ablation inductively coupled plasma mass spectrometry - Implications for mineral microanalysis

The silicate glass standard reference material NIST SRM 610 has been repeatedly analysed by UV and IR laser ablation ICP-MS in procedures designed to mimic typical analytical procedures adopted during mineral analysis. Trace element fractionation during both IR and UV laser ablation is observed and relationships between fractionation trends and ionic radius, charge and melting temperature of the elements determined are defined and discussed. In particular the high field strength elements (e.g., Mb, Ta, Zr, Hf) all show decreasing fractionation trends with respect to Si in the NIST SRM 610 glass during repeated laser ablation, whilst the low field strength and large ion lithophile elements (e.g., Rb, Sr, Pb, Ba, Ca) all show increasing trends with respect to Si. For IR laser ablation the degree of fractionation observed in the analysis of the REEs is strongly correlated to their ionic radii. In all the analytical procedures studied, fractionation arising from IR laser ablation is considerably greater than UV laser ablation and simple changes to analytical procedure to reduce trace element fractionation are suggested. For UV laser ablation, the effect of laser focus on analytical precision is assessed. Active focussing of the laser during ablation under computer control significantly improves analytical precision.