A local aerosol extraction strategy for the determination of the aerosol composition in laser ablation inductively coupled plasma mass spectrometry

Abstract

Aerosol transport efficiency in UV-ns LA-ICP-MS of less than 100% requires a representative aerosol composition for precise and accurate quantitative analysis. Therefore, aerosol expansion related changes in the composition of aerosols generated using a 193 nm excimer laser were studied within the ablation cell using an in-cell aerosol extraction strategy. The gas flow pattern within the ablation cell in the proposed local aerosol extraction was modelled using computational fluid dynamics techniques. Compared to commonly applied ablation cell geometry, the peak height of a single laser shot was increased by a factor of 13.5, the signal width was reduced by a factor of 12 and the washout time of the sample cell was consequently shortened to approx. 2 s, thereby almost eliminating processes of aerosol recirculation within the cell. The selective extraction of aerosol from different positions of the expanding laser plume was realized by subsequently changing the sampling distance between the ablation site and the gas outlet nozzle tip. The results show a similar distribution of siderophile elements (P, Cr, Mn, Fe, Co, Ni, Ga, Ge, Mo, W, Au), chalcophile elements (Cu, Zn, As, Se, Rh, Ag, Cd, In, Sn, Sb, Te, Pt, Tl, Pb, Bi) and some of lithophile elements (Li, B, Na, Mg, Si, K, V, Rb, Ba, U) within the expanding plume. In contrast, lithophile elements Be, Al, Ca, Sc, Y, Zr, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta and Th were significantly depleted towards increasing sampling distances from the ablation site. Furthermore, the plume composition depends strongly on the ambient gas used within the ablation cell. Evaluating various physical properties of individual elements it becomes clear that no single parameter dependence exists. However, elements with oxide melting points higher than 1500 °C tend to be depleted towards cooler regions of the expanding aerosol. The proposed local aerosol extraction strategy is suitable for the identification of position dependent and, therefore, an indirect indicator for particle size dependent elemental composition of 193 nm laser generated aerosols under He atmosphere, which could not be studied using particle separation devices. In contrast to the ablation in helium, the changes of the aerosol composition in argon were less variable amongst different elements when sampling at different distances from the ablation crater. However, Zn and Cd intensities increased when sampling further away from the ablation crater. Results indicate that aerosol expansion within the ablation in UV-ns laser ablation can be a significant source of non-stoichiometric sampling, especially induced by aerosol deposition on the sample surface.