Overcoming challenges in analysis of polydisperse metal-containing nanoparticles by single particle inductively coupled plasma mass spectrometry

Detection and sizing of metal-containing engineered nanoparticles (ENPs) was achieved at concentrations predicted for environmental samples (part-per trillion levels) using single particle inductively coupled plasma mass spectrometry (SP-ICP-MS). Silver nanowires, titanium dioxide and cerium oxide nanoparticles were found to be detectable by this technique, while zinc oxide dissolved too quickly for analysis at these concentrations. In addition to the potential dissolution of particles, other considerations affecting ENP analysis include: instrumental background, mass interferences, percent metal in a nanoparticle, and isotopic abundance of the analyte element. Sizing of these metal-containing nanoparticles was done by correlating ICP-MS response (pulses) from ENPs entering the plasma to mass of metal in dissolved standards. The resulting particle size distributions compared well with results from sedimentation field-flow fractionation. Coincidence in ENP pulses may be difficult to detect in the broad size distributions that arise from polydisperse samples. Comparison of data obtained by combination of multiple analyses of dilute solutions to single analyses of higher concentration allowed discrimination between coincidence and polydispersity. The ratio of ENP pulse detections to the total number of readings during analysis was optimized at 2.5% or less to minimize coincident pulses while still allowing definition of a size distribution.