Comparison of Ultrafine Powder Pellet and Flux-free Fusion Glass for Bulk Analysis of Granitoids by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry

Two sample preparation techniques (ultrafine powder pellet and flux-free fusion glass) for LA-ICP-MS bulk analysis of granitoids were compared. Ultrafine powder particles produced by wet milling were characterised as d(50): similar to 1.0 mu m, d(90): similar to 5.0 mu m. Agate abrasion (balls and vial) during wet milling affects only SiO2 measurements and is negligible for other elements. For the flux-free fusion glass, a second grinding of the initial fused glass and re-melting were necessary to produce compositionally homogeneous glasses. Nickel and Cu can be contaminated during the fusion procedure, and Sn and Pb were depleted after the melting process. The homogeneity of fusion glasses was comparable to that of MPI-DING glasses, while the powder pellets were less homogeneous. This heterogeneity is ascribed to large (up to 10 mu m) crystal fragments (e.g., biotite) persisting in powders even after 45 min of milling. For most elements of interest, both preparation techniques give reliable LA-ICP-MS results of granitoid reference materials within 10% of the reference values. Thus, we can recommend both techniques to avoid common problems associated with acid dissolution ICP-MS. For high-precision measurements (especially Zr, Hf, Th and U), the flux-free fusion glass technique is a better choice than ultrafine powder pellets.