Elemental fractionation in laser ablation-inductively coupled plasma-mass spectrometry: evidence for mass load induced matrix effects in the ICP during ablation of a silicate glass

Abstract

To study mass load dependent matrix effects in LA-ICP-MS, different masses of aerosols from the NIST 610 glass were generated using a 193 nm ArF excimer laser and a 266 nm Nd:YAG laser and introduced into an ICP-MS. The different mass loads in the ICP were achieved using different crater diameters, the application of an aerosol dilutor, a tandem ablation setup and mixing of a single element matrix (desolvated solution) with the laser generated aerosols. The comparison of results acquired using these different experimental setups supports the existence of a significant matrix effect dependent on the mass load of the ICP. The proof of such effects on 266 nm laser generated aerosols was limited by temporal changes of element ratios due to the generation of a broad particle size distribution and the related incomplete vaporization within the ICP. However, the differences in the particle size distributions measured for various crater diameters as well as the temporal changes of the element ratios using the 193 nm ArF excimer laser were insufficient to explain mass load related changes of the element ratios. It is shown that an increase of the mass load of the ICP by a factor of 16 (crater diameter from 30 to 120 μm) leads to a decrease in certain intensity ratios (e.g., Cu/Ca, Zn/Ca, Cd/Ca, Pb/Ca) up to 25%. Even after taking into account the fact that smaller crater diameters might be partially influenced by laser-induced fractionation, an excessive mass load of the ICP using two 193 nm laser ablation systems demonstrates a further decrease in ion signal intensities of volatile elements in comparison with Ca. In contrast, applying dilution up to a factor of 30 to the aerosols generated using different crater diameters leads to a stabilization of the intensity ratios (e.g. Cd/Ca) to a constant value. The mass load enhanced matrix effect is element dependent and most severe for elements with low melting points (e.g., Cu, Zn, Ag, Cd, Pb). Based on the changes of e.g. As/Ca, Sn/Ca and the constant Be/Ca ratios, an explanation of the plasma induced effects dependent on parameters such as 1st or 2nd ionization potential was impossible. Furthermore, the mass load dependence on easily ionizable elements (Cs, Rb) indicates the absence of incomplete vaporization related effects. In contrast, Al/Ca, Ti/Ca, Th/Ca ratios, for example, remained constant (within 2–3%) with and without dilution. The induction of matrix effects independently of the ablation process by adding various Rb concentrations to the laser aerosols indicates that elements (Cu, Zn, Cd, Pb, U) previously described to be dominantly influenced by laser-induced elemental fractionation undergo significant ICP-induced matrix effects. These matrix effects are mass load dependent and for most elements exceed the contribution of laser-induced fractionation.