Analyte- and matrix-dependent elemental response variations in laser ablation inductively coupled plasma mass spectrometry Part 1. The roles of plasma and ion sampling conditions

Abstract

Elemental response variations as a function of carrier gas flow rate in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were studied for a wide range of analytes. The effects of rf power, focus lens settings, thermodynamic properties of analytes and sample matrix were thoroughly examined. It was found that, with the experimental set-up used for this work, processes occurring in the ICP, rather than during ablation and transport, play the decisive role in determining the shapes of flow rate plots observed with LA. Responses for analytes of lower nominal masses and vaporization enthalpies peak at consistently higher flow rates (1.15 l min⁻¹) than other elements, independent of matrix. On the other hand, the magnitude of the maximum sensitivity is matrix dependent, even for these elements. Involatile elements display much broader maxima at considerably lower flow rates; the more refractory the matrix, the lower the optimum flow rate. This behaviour is consistent with the residence time in the ICP necessary to maximize the efficiency of analyte ion production. The existence of inter-elemental differences in the locations of zones of maximum ion densities formed in the ICP can thus be related to the times required for vaporization of any given analyte from the particles produced by LA. Such differences may be responsible for numerous fractionation effects mentioned in the LA literature. It is also demonstrated that the ion sampling process can affect the shapes of the flow rate plots, potentially shifting the apparent position of the optimum flow rate and confounding the interpretation of inter-element response differences.