SI-traceable characterisation of the first reference material for nanoparticle number concentration in suspension to support regulatory compliance.

Reference materials (RMs) are increasingly needed to support number-based characterisation of nanomaterials (NM) in a regulatory context for the purpose of method development, validation and measurement quality control. To date, RMs for number concentration in suspension with a directly assigned value that is SI traceable have been rather scarce, being the LGCQC5050 the only material commercialised so far. This could be attributed to the limited availability of metrologically validated measurement methods and stability challenges associated with long-term storage of NM suspensions. This paper describes development and characterisation of the first RM consisting of 30-nm colloidal gold nanoparticles and value assigned for particle number concentration using the dynamic mass flow (DMF) method with single particle ICP-MS (spICP-MS). Special attention is paid to systematic assessment of the DMF method's performance under operating conditions set outside recommendations described in ISO/TS 19590:2024, but that still comply with key requirements of this method (e.g. use of an ICP-MS system in equilibrium, a cooled spray chamber, etc.). The results of such investigations are reported here for the first time. The paper also discusses practical considerations for the production, storage and transport of nano RMs, and provides guidance on best practice for the production and certification of future nano RMs in accordance with ISO 17034:2016.