Measurement and calibration strategy for ICP-MS single-particle analysis

Abstract

Calibration of ICP-MS single-particle analysis using standard particles of known particle mass is required for accurate measurement. Calibration using the average particle mass of a batch of standard particles is effectively a single-point calibration. In this study, the feasibility of constructing a calibration curve by correlating the distribution of ICP-MS spike intensity and the distribution of the mass of standard particles is investigated. Gold nanoparticles are used as the test particles. The particle size is determined using TEM. The particle mass is calculated from the volume and density of the particle. The mass range of monodisperse gold nanoparticles is relatively small, spanning over approximately a factor of 2. Linear calibration curves for Au nanoparticles of nominal diameter of 80, 100, 150, and 200 nm are obtained. The slope of the calibration curves, however, reduces as the diameter increases. The combined calibration curve of all 4 batches of standard particles is non-linear although the mass range spans over a factor of 30 only. The limited linear dynamic range is probably combined effect of incomplete vaporization of the large particles and relatively significant diffusion loss of analyte atoms for the small particles. A computer program that accounts for the rate of particle vaporization and analyte atom diffusion is used to estimate the relative ICP-MS intensity of standard particles at different ICP sampling depth. Simulated calibration curves of 4 kinds of hypothetical nanoparticles representing 4 combinations of boiling point and analyte diffusion rate will be presented. The relative contribution of the rate of particle vaporization and analyte diffusion on the linearity and slope of the calibration curve will be discussed. The conditions for satisfactory calibration using continuous flow of aerosols of standard solutions will also be discussed.