Validation of single particle ICP-MS for routine sizing and quantification of the fraction of silver nanoparticles in E174 food addititves and confectionery products

Silver (Ag) is a food additive (E174) approved by the European Commission to be used for the external coating of confectionery, for decoration of chocolates, and in liqueurs [1]. It is commercially distributed in its pristine powder and sheet form, and in confectionery products. Due to its nature, E174 may contain silver nanoparticles, which implies a need for validated methods to size and quantify these particles. Single particle inductively coupled plasma-mass spectrometry (spICP-MS) is thereby a promising technique as it is capable of sizing and counting particles at the same time.
A spICP-MS method was developed and validated for sizing and quantifying the fraction of silver nanoparticles in E174 food additives and in products containing E174. The samples were prepared for analysis according to a slightly modified version of the method of Jensen et al. [2]. The E174 food additives showed a large silver background concentration combined with a relatively low number of nanoparticles, making the quantification of the nanoparticles more challenging than in the products containing E174. Validation of the method showed good performance with respect for the size distribution compared to the size distribution obtained from transmission electron microscopy. Depending on the sample and the background silver concentration, particles with an equivalent spherical diameter (ESD) down to 11 nm could be detected. The performance in terms of repeatability (size 4-11%, concentration 16-29%), and intermediate precision (size 2-8%, concentration 18-31%) depended on the type of sample. The large repeatability compared to the intermediate precision demonstrates the need to analyze multiple independent replicates under routine conditions. When analyzing three replicates, the extended measurement uncertainty (k = 2) on the mean ESD is 20% for E174 food additives and 11% for products containing E174. The quantification of the mass and number concentration is more challenging with extended measurement uncertainties up to 45%.