Laser ablation-ICP-MS: particle size dependent elemental composition studies on filter-collected and online measured aerosols from glass

Abstract

In laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), the term elemental fractionation is normally used to summarize all non-stoichiometric effects occurring during sample ablation, aerosol transport and vaporization, atomisation and ionisation within the ICP. Nevertheless, there are different types of elemental fractionation occurring between different sized particles within an aerosol, as previously shown for metal alloy ablations. In the present work, laser generated aerosols from glass samples were filter collected before entering the ICP to study their total and particle size dependent composition. Furthermore, elemental concentrations in different particle size fractions on filters were compared with their response measured in the ICP. For the NIST SRM 610 glass, elemental fractionation effects between small particles (<125 nm and <340 nm), and the total aerosol containing all particle sizes up to 1 µm or even larger, were measured for 42 major and trace elements using a 266 nm Nd∶YAG laser and scanning ablation conditions to produce the aerosol. Particles above 125 nm and 340 nm were separated from the aerosol by a particle separation device and the remaining particles were collected on a filter, digested and measured using solution nebulization ICP sector field MS (SFMS). Results show an enrichment of certain elements such as Cu, Zn, Ag, Tl, Pb and Bi of up to 90% relative to Ca within the small particle size fraction of an aerosol in comparison with the total composition of the aerosol. The same elements are depleted in large aerosol particles measured from deposited particles within the ablation cell. However, the total transportable aerosols produced using different laser wavelengths (193/266 nm), and gas environments (He/Ar), which were also filtered and digested show no significant deviation in their overall stoichiometry from the original sample (except Be, Fe, Cd for all lasers and gases) within the uncertainty of the measurements. Therefore, the composition of filter collected aerosol of glass samples indicates that the elemental fractionation in LA-ICP-MS, detected at the beginning of a 266 nm single hole ablation, is predominantly caused by incomplete vaporization of large particles within the ICP and is not dominated by non-stoichiometric ablation of the glass.