This document specifies a method for the detection of nanoparticles in aqueous suspensions and characterization of the particle number and particle mass concentration and the number-based size distribution using ICP-MS in a time-resolved mode to determine the mass of individual nanoparticles and ionic concentrations. The method is applicable for the determination of the size of inorganic nanoparticles (e.g. metal and metal oxides like Au, Ag, TiO2, BVO4, etc.), with size ranges of 10 nm to 100 nm (and larger particles up to 1 000 nm to 2 000 nm) in aqueous suspensions. Metal compounds other than oxides (e.g. sulfides, etc.), metal composites or coated particles with a metal core can be determined if the chemical composition and density are known. Particle number concentrations that can be determined in aqueous suspensions range from 106 particles/L to 109 particles/L which corresponds to mass concentrations in the range of approximately 1 ng/L to 1 000 ng/L (for 60 nm Au particles). Actual numbers depend on the type of mass spectrometer used and the type of nanoparticle analysed. In addition to the particle concentrations, ionic concentrations in the suspension can also be determined. Limits of detection are comparable with standard ICP-MS measurements. Note that nanoparticles with sizes smaller than the particle size detection limit of the spICP-MS method may be quantified as ionic. The method proposed in this document is not applicable for the detection and characterization of organic or carbon-based nanoparticles like encapsulates, fullerenes and carbon nanotubes (CNT). In addition, it is not applicable for elements other than carbon and that are difficult to determine with ICP-MS. Reference  gives an overview of elements that can be detected and the minimum particle sizes that can be determined with spICP-MS.