Release of TiO2 and ZnO nanoparticles from sunscreens into natural waters: detection and discrimination from natural particles using SP ICP-ToF-MS

From an ecological risk perspective, it is important to differentiate engineered nanoparticles (ENPs) from naturally occurring nanoparticles (NNPs). The aim of this research was to characterize and quantify titanium dioxide and zinc oxides nanoparticles (NPs) that were released from two commercial sunscreens into three aqueous matrices (ultrapure, hard and soft natural waters) after two short term exposures: ∼15 min and ∼60 min. An inductively coupled plasma time-of-flight mass spectrometer (ICP-ToF-MS) was used to detect elements with mass to charge (m/z) ratios ranging from 26 to 210 amu within single particles (SP). The elemental compositions, mass distributions and isotopic ratios (47Ti/49Ti and 66Zn/68Zn) of the individual NPs were investigated in order to determine to what extent it was possible to discriminate the natural and engineered NPs. The coupling of an ion-exchange resin to the ICP-ToF-MS resulted in a reduced background signal for zinc, leading to the detection of reasonably small zinc oxide nanoparticles (size detection limit of ∼53 nm on the ICP-ToF-MS). For both commercial sunscreens, Zn was primarily released as dissolved forms, with nearly all of the Zn found below the size detection limits or adsorbed to NNP after 60 minutes. Based upon the SP-ICP-ToF-MS results, the detected NPs in the sunscreens mainly contained single elements, in contrast with the natural NPs. Elemental ratios were helpful to distinguish the ENP from NP, but isotopic ratios (Ti or Zn) were not a distinguishing factor for the NP, in this case. Spearman rank analysis provided an additional index to distinguish the different particle types.