Lowering the Size Detection Limits of Ag and TiO₂ Nanoparticles by Single Particle ICP-MS

As the production and use of engineered nanomaterials increase, there is an urgent need to develop analytical techniques that are sufficiently sensitive to be able to measure the very small nanoparticles (NP) at very low concentrations. Although single particle ICP-MS (SP-ICP-MS) is emerging as one of the best techniques for detecting NP, it is limited by relatively high size detection limits for several NP, including many of the oxides. The use of a high sensitivity sector field ICP-MS (ICP-SF-MS), microsecond dwell times, and dry aerosol sample introduction systems were examined with the goal of lowering the size detection limits of the technique. For samples injected as a wet aerosol, size detection limits as low as 4.9 nm for Ag NP and 19.2 nm for TiO₂ NP were determined. By using a dry aerosol, a significant gain in ion extraction from the plasma was obtained, which resulted in a noticeable decrease of the size detection limits to 3.5 nm for the Ag NP and 12.1 nm for the TiO₂ NP. These substantial improvements were applied to the detection of TiO₂ NP in sunscreen lotions, rainwaters, and swimming pool waters. Concentrations of Ti-containing NP between 27 and 193 μL^-1 were found in rain samples. Similar NP concentrations were detected in public swimming pools, although much higher particle number concentrations (6046 ± 290 μL^-1) were measured in a paddling pool, which was attributed to a high concentration of sunscreen lotions in a small recirculated water volume. High losses of TiO₂ NP through adsorption or agglomeration resulted in recoveries ranging from 14-34%.