2.2. Qualitative Assessment of the Number Size Distribution with spICP-MS

As a first qualitative assessment before shipping the test materials to the laboratories participating in this study, the number size distribution of the tested materials was determined with spICP-MS. To this end, first the transport efficiency was determined based on the TES method, followed by re-measuring the same suspension as a sample. Tested AuNP suspensions were diluted to reach approximately 100,000 particles mL⁻¹. Solutions of dissolved gold (blank and four solutions ranging from 1 µg L⁻¹ to 5 µg L⁻¹) were prepared by diluting the stock solutions with ultrapure water. Ionic gold standard solutions for ICP-MS (1 g L⁻¹ in 5% HCl) were purchased from Sigma-Aldrich (St. Louis, MO, USA).

A Perkin Elmer NexIon 300D quadrupole ICP-MS, equipped with an SC Fast peristaltic pump, a Meinhard concentric nebuliser, a glass cyclonic spray chamber and a standard quartz torch (2.0 mm injector i.d.) operating in standard mode was used for spICP-MS analysis (Perkin Elmer, Waltham, MA, USA). Operating conditions were checked daily to achieve maximum sensitivity. For the setting of all parameters and data acquisition, the Nano Application Module of Syngistix™ software version 2.5 was used. The dwell time was set at 100 μs and the total data acquisition time at 60 s. The exact flow rate of the peristaltic pump required for the determination of the transport efficiency was measured gravimetrically daily and ranged between 0.15 mL min⁻¹ and 0.18 mL min⁻¹. For the determination of AuNP suspensions, the gold-197 isotope was monitored setting the mass fraction and the ionisation efficiency to 100% and density to 19.3 g cm⁻³.