Accurate Measurement of Ultrafine Silica Nanoparticles Using ICP-MS/MS

Silica (SiO2) nanoparticles (NPs) are widely used in industry, manufactured goods, and consumer products, such as food additives. However, there are several significant challenges associated with the measurement of silicon by inductively coupled plasma-mass spectrometry (ICP-MS). First, silicon has a moderately high first ionization potential (8.152 eV), which means it is less than 70% ionized in a typical argon plasma, so the sensitivity is significantly lower than for a fully ionized element. In addition, background polyatomic ions affect all three silicon isotopes, 28Si, 29Si, and 30Si, with the most intense interferences (14N2+ and 12C16O+) occurring on the major 28Si isotope. Finally, silicon-based materials are widely used in manufactured goods and industrial materials ranging from glass, alloys, and air and water filters to organic compounds such as silicone tubing, sealants, adhesives, lubricants, and coatings. Consequently, maintaining a low background for silicon in the analytical laboratory can be highly problematic. When characterizing silica in nanoparticle form, the analysis is especially difficult. In this article, we describe a method to measure ultrafine silica nanoparticles using ICP-tandem MS (ICP-MS/MS) to control the elemental and polyatomic ion backgrounds. The method provides high sensitivity to ensure accurate analysis at the small (~50 nm) particle sizes of interest in environmental and biological applications.