Study of the influence of water vapour loading and interface pressure in inductively coupled plasma mass spectrometry

Abstract

The influence of water vapour loading on ion counts for singly charged, doubly charged, oxide and hydroxide species has been studied for ten elements (Li, Mg, As, Mo, Cd, Cr, W, Co, Ba and Pb) covering both a wide mass range from ⁷Li to ²⁰⁸Pb and a wide range of oxide and hydroxide species stabilities. Studies have been carried out using identical sample introduction systems for two different inductively coupled plasma mass spectrometry (ICP-MS) instruments (the VG PlasmaQuad and the Sciex Elan), and the behaviour of the two instruments has proved to be substantially different in this respect. Measurements of water loading as a function of spray-chamber temperature have shown that the nebuliser gas is precisely saturated with water vapour at the temperature of the spray chamber. Electron density and excitation temperatures of the plasma, measured as a function of water vapour loading in the plasma, have shown little variation with water vapour loading over the range examined. Studies of ICP-MS interface pressure as a function of water vapour loading indicate that variations in this loading may influence the partial pressures of species present in the interface sufficiently to alter both ion trajectories in the interface and ion movement in the ICP-MS lens system.