The effect of carrier gas humidity on the vaporization of laser-produced aerosols in inductively coupled plasmas

Abstract

In a recent study we reported on the discovery of an interrelationship between the kind of material sampled by laser ablation (LA) and the shape of optical emission spectrometry (OES) profiles formed by atoms along the axis of an inductively coupled plasma (ICP), which is commonly used as an atomization and ionization source for mass spectrometry (MS). These results encouraged us to resume efforts on OES-based diagnostics to investigate vaporization-related elemental fractionation effects occurring during LA-ICP-MS and to conceive strategies for their suppression. Since the quantification capabilities and sensitivity of LA-ICP-MS tend to improve when water is simultaneously added to the carrier gas, we hypothesized that a matching of analyte/material-dependent points of vaporization in the ICP may be responsible for the increased accuracy that is often observed under such conditions. In this work, the impact of water admixture was investigated by side-on OES of an ICP using a Czerny–Turner monochromator operating in 2D imaging mode. Our data indicated a superposition of calcium- and sodium-specific OES axial profiles which were separated by several millimeters when no water was supplied, thus, supporting the hypothesis previously made. Furthermore, the utilization of a micro-droplet dispenser allowed to precisely adjust the amount of water entrained into the ICP and to specify the range of relative humidity required for matching the points of vaporization. Conditions specified this way were applied to quantitative LA-ICP-MS analyses of silicate glass and brass using matrix matched and non-matrix matched calibration, respectively, whereby the former resulted in significant improvements in the accuracy as well as lower detection limits with only moderate increases of the oxide formation rate.