Unraveling the role of aerosol transport on nanomaterial characterization by means single particle inductively coupled plasma mass spectrometry

Abstract

Nanomaterials (NMs) characterization by means single particle inductively coupled plasma mass spectrometry (spICP-MS) relies on assessing transport efficiency. To this end, different strategies have been proposed but contradictory reports could be found about which strategy provides the most accurate results since our knowledge about NMs transport through the sample introduction system is still limited. The goal of this work is to evaluate the role of aerosol transport on NMs characterization by means spICP-MS. To this end, a 70 nm platinum nanoparticles (PtNPs) suspension and a 10 ng mL⁻¹ ionic Pt solution were analyzed under different operating conditions. Next, plasma and tertiary aerosol characteristics (i.e., drop size distribution and transport rate of solvent, PtNPs and Pt ions) were checked to explain experimental findings. Our results shows that the number of events, PtNPs event intensity and Pt ionic signal depend on both aerosol transport and plasma operating conditions. Interestingly, tertiary aerosol characterization reveals that NMs, ionic and solvent transport efficiencies differ significantly. Thus, irrespective of the nebulization conditions, transport efficiencies follow the order solvent > ionic Pt > PtNPs. For instance, when operating a nebulizer gas flow of 0.9 L min⁻¹ and a sample uptake rate of 300 μL min⁻¹, transport efficiency values were 5.77 ± 0.03, 3.89 ± 0.12 and 3.35 ± 0.06%, respectively. Similar results were observed operating different metallic NPs (i.e., 50/150 nm AuNPs) and spray chamber designs (i.e., Scott double pass/cyclonic spray chambers). These findings are of fundamental importance for spICP-MS metrology since some strategies for evaluating transport efficiency are based on assuming that the above-mentioned species are transported similarly into the plasma. The method based on the number of events seems the best approach since it is really based on NMs transport into the plasma. Both ionic and NMs pulse intensity ratios and solvent-based methodologies are, however, not based on NMs transport efficiency and, hence, they could lead to inaccurate NMs characterization in terms of particle size and concentration depending on spICP-MS operating conditions. For both approaches, because the dampening effect of cubic root when calculating particle size from transport efficiency, particle size distribution bias was less significant than particle concentration bias.