Characterization of SiO2 nanoparticles by single particle-inductively coupled plasma-tandem mass spectrometry (SP-ICP-MS/MS)

Abstract

The increase in the use of SiO₂ nanoparticles (NPs) is raising concern about their environmental and health effects, thus necessitating the development of novel methods for their straightforward detection and characterization. Single particle ICP-mass spectrometry (SP-ICP-MS) is able to provide information on the size of NPs, their particle number density and mass concentration. However, the determination of Si via ICP-MS is strongly hampered by the occurrence of spectral overlap from polyatomic species (e.g., CO⁺ and N₂⁺). The use of tandem ICP-MS (ICP-MS/MS) enables interference-free conditions to be obtained, even in the most demanding applications. Upon testing several gases, the use of CH₃F (monitoring of SiF⁺, mass-shift approach) and of H₂ (monitoring of Si⁺, on-mass approach) were demonstrated to be the most suitable to overcome the spectral interference affecting ultra-trace Si determination (LoD < 15 ng L⁻¹). By using these approaches, SiO₂ NPs (ranging between 80 and 400 nm) can be detected and characterized. For SiO₂ NPs > 100 nm, it was possible to provide accurate results in a straightforward way, as the signals they give rise to are well resolved from those of the background. In the case of 80 and 100 nm NPs, the use of a simple deconvolution approach following a Gaussian model was needed to characterize SiO₂ NPs apparently showing incomplete distributions as a result of the presence of the background signal. Overall, the methods developed using SP-ICP-MS/MS are sensitive and selective enough for the interference-free determination of Si at ultra-trace levels, also in the form of SiO₂ NPs.