Use of Aerosol Dilution to Improve Characterization of ZnO Nanoparticles by Single Particle Inductively Coupled Plasma Mass Spectrometry

J. Liu, V.A. Hackley
National Institute of Standards and Technology,
United States

Keywords: Single particle ICP-MS, characterization, ZnO nanoparticle, aerosol dilution

Summary:

Risk assessment for the manufacture and application of engineered nanomaterials (ENMs) in consumer and medical products demands advanced analytical methods for characterizing and quantifying the exposure to ENMs and their progeny in environmental and biological systems. Recently, methodologies involving single particle inductively coupled plasm mass spectrometry (spICP-MS) have advanced rapidly for characterization of the elemental composition, size, number concentration and particle/ion partitioning of metal containing nanoparticles (NPs) under environmentally relevant conditions. However, detecting and accurately sizing ENMs (e.g., ZnO, Ag) that undergo significant dissolution at the very low concentrations used in spICP-MS remains a significant challenge. In this study, we have developed an aerosol dilution method for in situ characterization of ZnO NPs by spICP-MS. Aerosol dilution was achieved using an ultra-high matrix introduction (UHMI) system, which dilutes the sample aerosol as it passes to the plasma by adding a second argon gas flow between the spray chamber outlet and troch. The effect of aerosol dilution on spICP-MS performance was first evaluated using AuNP reference materials. The dilution factor is controllable up to 200-fold by adjusting the nebulizer and dilution gas flow. We found the sensitivity on unit mass of Au is nearly independent of the UHMI conditions, whereas the transport efficiency decreases upon increase of dilution gas flow. Thus, the contribution of dissolved species to the baseline signal was greatly diminished without sacrificing NP intensity. Use of aerosol dilution to improve the limit of size detection was demonstrated in AuNP suspensions spiked with Au+. Furthermore, two ZnO NP test samples were characterized by spICP-MS under selected UHMI conditions. Sample preparation, which can modify the stability of ZnO in aqueous suspensions, was found to be critical and was therefore evaluated to achieve improved detection and characterization of the size distribution.