Chemical and phase composition of particles produced by laser ablation of silicate glass and zircon—implications for elemental fractionation during ICP-MS analysis

Abstract

The chemical and phase compositions of particles produced by laser ablation (266 nm Nd:YAG) of silicate NIST glasses and zircon were studied by SIMS and HR-TEM techniques. The data suggest that the formation of phases of different mineralogy and/or chemical composition from the original sample at the ablation site can result in elemental fractionation (non-stoichiometric sampling) in material delivered to the ICP-MS for quantitative analysis. Evidence of the element fractionation is preserved in chemically zoned ejecta deposited around the ablation pit. The chemical composition and mineralogy of particles varies with particle size so that the efficiency of transport of particles also plays a role in elemental fractionation. During the first 250 pulses in a typical ablation experiment using a 266 nm laser, particle sizes are mainly o2.5 mm; thereafter they decrease to o0.3 mm. Pb and U are fractionated significantly during the ablation of both silicate glass and zircon. During the ablation of glass, both micron-sized, melt-derived, spherical particles, and nm-sized, condensate-derived particle clusters, are produced; the very smallest particles (o0.04 mm) have anomalously high Pb/U ratios. For zircon, both larger (0.2-0.5 mm) spherical particles and agglomerates of smaller (B0.005 mm) particles produced by ablation are mixtures of amorphous and crystalline materials, probably zircon, baddeleyite (ZrO 2 ) and SiO 2 . Evidence for thermal decomposition of zircon to baddeleyite and SiO 2 is preserved in the wall of the ablation pit, and may lead to the commonly observed increase in Pb/U recorded during laser ablation ICP-MS analysis. It follows that a matrix-matched external calibration is essential for achieving highly precise and accurate laser (266 nm wavelength) ablation ICP-MS analysis of Pb and U in silicate samples.

Matrix-independent Fe isotope ratio determination in silicates using UV femtosecond laser ablation

“…Well-known matrix-dependent processes for ns LA are 1) fractionation at the ablation site, which leads to non-stoichiometric aerosols (e.g. Eggins et al, 1998;Hergenröder, 2006;Košler et al, 2005) and 2) particle-size-related fractionation due to incomplete transport and ionisation of large particles in the ICP (e.g. Koch et al, 2002;Guillong et al, 2003).…”

Applications of LA-ICP-MS in the elemental analyses of geological samples

“…Elemental fractionation can occur during the ablation process, the aerosol transport process or the excitation process in ICP [86,[96][97][98][99][100][101]. The intensity of the fractionation effect caused by temporal changes in elemental responses during laser ablation can be measured by the fractionation index (FI):…”

Femtosecond laser ablation inductively coupled plasma mass spectrometry: achievements and remaining problems

“…Apparently, these elements show a high degree of thermal volatility, low melting points, and partially different ionization potentials and are therefore extremely sensitive to any changes of the LA and ICP conditions. Considering, for instance, the Pb-and U-specific isotope analysis of zircons, which is often used as a chronometer for geological events (geochronology), temporal drifts of the Pb/U ratio were found to restrict the accuracy [9,10]. It should be emphasized that such fractionation-related inaccuracies can largely be avoided if matrix-matched standards are used for calibration.…”