Real-time characterization of particles produced by laser ablation for analysis by inductively coupled plasma mass spectrometry

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, using a glass standard and a pitchblende ore as samples. 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. This approach can be extended to other samples and laser wavelengths to comprehensively characterize LA-generated particles and their effects on corresponding ICPMS signals.