Phase and composition changes of titanite during laser ablation inductively coupled plasma mass spectrometry analysis.

Changes in phase and chemical composition of the mineral titanite (monoclinic CaTiSiO(5)) during laser ablation and plasma-aerosol interaction were investigated using electron diffraction and electron microbeam X-ray analysis with transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Laser ablation of titanite with a solid state 213 nm nanosecond laser generates a bimodal aerosol consisting of condensed nanoparticles and spherical micrometer-sized particles. The two types of particles produced by laser ablation are amorphous on the scale resolvable by the electron diffraction. The ratio of Ca-Si-Ti does not change significantly during laser ablation. Aerosol of titanite particles introduced into the ICP and collected after interaction with the plasma contains nanometer-sized particles of a condensate and spherical micrometer-sized particles with a molten surface. The condensed particles are enriched in silicon whereas the spherical micrometer-sized particles show a deficiency in Si relative to the titanite composition. During laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analysis of titanite, Si and Ti showed positive and negative fractionation trends relative to Ca, respectively. This is consistent with the observed chemical composition changes of the titanite aerosol within the ICP. This study links for the first time the chemical and phase changes of a sample within the ICP to the elemental fractionation during LA-ICPMS.

Effect of oxygen on laser-induced elemental fractionation in LA-ICP-MS analysis.

Elemental fractionation poses serious difficulties in obtaining accurate concentration and isotope ratio data when using laser ablation sampling. One of the factors that control the extent of laser-induced elemental fractionation is the composition of sample carrier gas in the sample cell. This study demonstrates that the presence of small amounts of oxygen in the He carrier gas has a significant effect on elemental fractionation during the ablation of silicate (NIST 612 glass and zircon 91500) and sulphide (NiS fire assay) samples. The extent of elemental fractionation for a given amount of ablated material and concentration of oxygen in the He carrier gas was related to the volume of the plasma plume that forms above the sample surface. This indicates that an oxidation reaction takes place in the plasma plume. It has been reported that oxidation can affect the particle size distribution during laser sampling and hence change the extent of elemental fractionation. The purity of the carrier gas used in laser ablation-ICP-MS, as well as the amount of oxygen released from silicate and oxide samples during the ablation in "oxygen-free" ambient gas, is shown to contribute significantly to elemental fractionation.