PNNL

Abstract

Laser ablation (LA) inductively coupled plasma mass spectrometry (ICPMS) is a powerful technique used for trace element analysis of solid samples. While quantitative analysis can be achieved for many samples with careful use of matrix-matched standards, the complex relationship between the properties of the LA-generated particles and their dependence on the operating conditions, the vagaries of particle transport in the cell and transfer system, and the preferential or incomplete particle evaporation, atomization and ionization in the ICP limits this technique from being fully quantitative over a wide range of sample types and or analytical conditions. The characterization of LA-generated particles represents a particular challenge because of their complex shapes, morphologies, and large and size-dependent void fractions. Improved understanding of how laser settings affect the detailed properties of the LA-generated particles and how particle properties relate to ICPMS signal can potentially lead to ways to effectively tailor or modify inherent sample particles or to add engineered particles as standards, tracers, modifiers, carriers, or concentrators. This work investigates changes in physicochemical properties of LA-generated particles in response to altered laser ablation conditions, including laser power, repetition rate, flow rate, scan rate, and spot size. This is achieved using a multidimensional particle characterization approach that measures in real-time number concentrations, mobility and vacuum aerodynamic diameter distributions, mass, chemical composition, and effective density of individual particles as a function of LA conditions. These measurements yield fractal dimension, average diameter of primary spherules, number of spherules, void fraction, and dynamic shape factors as a function of particle mass or size. To our knowledge, this is the first such study to comprehensively examine these particle characteristics specifically for LA-ICPMS applications. For a NIST standard reference material (SRM) Glass 610 glass sample, particle size distributions significantly change with changing LA conditions and concentrations spanned two orders of magnitudes between high and low laser powers. All laser conditions produce highly irregularly shaped filament-like particles that are comprised of agglomerated nanoparticles with average diameters between 12 and 17 nm, depending on LA conditions. Particle effective density decreases with increasing mobility diameter/mass, but even for the smallest particles, density is significantly lower than their material density due to large particle void fractions. We find that particles with mobility diameters of 30 nm have ~25% voids, whereas 300 nm particles that are comprised of more than 1000 primary spherules have ~90% voids. Finally, we explore the use of graphitic agglomerated nanoparticles, produced by a spark discharge generator, using particles as high surface area analyte carriers to capture the ablated gas and particle soil material and transmit it to the ICPMS to increase its sensitivity. This paper serves to introduce this characterization method and establish its efficacy in identifying particle characteristics that are dictated by LA conditions. This can be extended to other samples and laser wavelengths to comprehensibly characterize LA-generated particles and to develop an understanding of the effects of particle mass, shape, and morphology on particle “critical” size and ICPMS signal.