Time-resolved measurements with single droplet introduction to investigate space-charge effects in plasma mass spectrometry

Abstract

An investigation of the space-charge induced effects of high concentrations of Pb⁺ matrix ions on Li⁺ analyte ions in inductively coupled plasma mass spectrometry (ICP-MS) is presented using a vertically oriented mass spectrometer with single droplet introduction. Greater reproducibility and stability in droplet-to-droplet sample introduction using the monodisperse microparticulate injector (MDMI) was achieved with the vertical orientation. Typical variation (%RSD) in the droplet-to-droplet arrival times, and mass spectrometry peak analytical areas are better than 5%. With this precision, a more quantitative description of the space-charge effect on a single cloud of ions is obtained. Both radial and axial space-charge effects were found to occur in the ion beam. Radial effects result in a loss in intensity because of poor transmission or collisions at surfaces within the mass spectrometer. Axial effects modify the kinetic energy distribution of background ion beam components (e.g., ¹⁶O⁺ and ⁴⁰Ar⁺) and sampled ion cloud constituents (e.g., ⁷Li⁺). However, axial effects do not appear to generate significant broadening of sampled ion clouds within the mass spectrometer. At the point of charge separation and high ion-beam charge density, the ion cloud maxima for Li and Pb are not coincident. This is because of mass dependent diffusion in the ICP as the ion clouds approach the sampling orifice. Space-charge induced ion loss occurs predominantly at a localized region after the Li⁺ sampled cloud peak maximum. When the Pb concentration in the sample is sufficiently high the ⁷Li⁺ sampled signal has a bimodal peak shape. The existence of the dip and its relative location in the bimodal ⁷Li⁺ sampled signal suggests that space-charge effects are localized to the region of high charge density occurring just after charge separation.