Zirconium interferences on the detection of silver nanoparticles by single particle ICP-MS: implications on natural water analysis

Background: The analysis of silver nanoparticle (NP-Ag) by the single particle technique with argon plasma-coupled mass spectrometry (SP-ICP-MS) is an increasingly used analytical approach. The sensitive technique, distinguishing particle size distribution, allows working at concentrations similar to those found in environmental samples. The two natural Ag isotopes 107 and 109, with abundances of 52 and 48% respectively, have similar sensitivity in ICP-MS detection. However, it is common to encounter isobaric interferences in mass spectrometry, and the element silver is not an exception, as much with the 107 isotope as 109. For both isotopes, zirconium oxides present isobaric interferences, either 91Zr16O, 90Zr16O1H for the isotope 107 and the 92Zr16O1H for the 109. Results: For surface water analysis by ICP-MS in regular technique, these interferences do not generally impact the analysis of total Ag concentrations (mainly dissolved) as they can be then simply subtracted like background signal. On the other hand, detection of NPAg was impacted by the interfering colloidal Zr. The analysis of Zr by the SP-ICP-MS technique of surface waters showed the presence of colloidal Zr, a random signal that cannot be simply subtracted from NP Ag signal. Our results show that, Zr colloids are effectively interfering with the NP-Ag assays by SP-ICP-MS technique where interferences translated into a false positive. Conclusion: The analytical issue related isobaric interferences from the naturally occurring colloidal Zr was attenuated (up to 250% in this assay) by the use of the 109 isotope in the Ag detection, limiting false positive detections and improving the reliability of NP-Ag measurements in natural waters. Therefore , more specific detection of NP Ag in surface waters that naturally contain Zr colloids can be accomplished.