Effect of particle size distribution on ICP-induced elemental fractionation in laser ablation-inductively coupled plasma-mass spectrometry

Abstract

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) has become one of the well-accepted analytical techniques for in situ trace element analysis and a large number of successful applications have shown its potential. Each commonly employed laser wavelength (1064, 266, 213, 193, 157 nm) leads to some degree of non-stoichiometric ablation, which makes quantification using non-matrix-matched calibration standards difficult for some elements. Time-dependent changes in elemental ratios (so-called elemental fractionation) have been ascribed mostly to processes occurring at the ablation site. Therefore, wavelengths and related irradiance are the major variables that have been used to study this phenomenon in detail. However, there are a large number of parameters that influence the ablation process, aerosol transport, and the excitation process within the ICP. Each process can contribute to elemental fractionation, making the effects of each difficult to separate and to study in detail. The influence of the ICP as one possible source has not been studied thoroughly. The aim of this study was the determination of the source of elemental fractionation using a 266 nm Nd∶YAG laser ablation system. The sample transport system was designed to keep gas flows and plasma conditions constant. Various ablation procedures (single hole drilling and scanning) were tested to investigate the influence of the particle size on the excitation process within the ICP. Mineral wool was used to filter various fractions of the laser-induced aerosol to study signal behaviour as a function of the mass load of the ICP. Uranium and thorium, two elements with very similar properties (ionisation potential and concentration) in the NIST 600 Glass standard series, were used in particular to study ICP processes. It is shown that the particle size distribution is dependent on the wavelength of the laser and the absorption behaviour of the sample. The 266 nm Nd∶YAG laser produces a particle size distribution which is significantly larger in comparison with aerosols produced using a laser wavelength of 193 nm. Signals related to the ablated volume show that the larger particle fractions are not completely vaporised and ionised in the ICP. Filtering certain particle fractions allows final stoichiometric excitation and ionisation, but is accompanied by a loss of 50–80% of the total signal. For single hole ablation, the particle size distribution becomes smaller with increasing depth of the crater. Therefore, scanning mode ablation (which takes place always at the surface) produces a constant supply of larger particles, which results in significantly higher matrix effects within the ICP, as shown by significant changes in the elemental ratio of U∶Th. These studies indicate that the secondary effect of incomplete aerosol or particle excitation within the ICP is the dominant process influencing elemental fractionation during LA-ICP-MS. The effect was observed to be different for individual ICP sources and, therefore, the requirement for matrix-matched quantification in LA-ICP-MS remains instrument-dependent.