The diagram above illustrates the workflow for Work Package 3 with partners’ responsibilities colour coded. This diagram is regularly updated as activities are completed (marked with ticks). The aim of this work package is to develop accurate methods to measure the surface chemistry and thickness of surface coatings on particles. A key objective is to develop particle reference materials which are spherical and monodisperse in both core diameter and shell thickness. These reference materials will be characterised by TEM and/or ASAXS to measure and verify these dimensions, and by ICPMS and GIXRF to determine the total elemental compositions. ICPMS is suitable for most metals and metalloids, rare earth elements, halides, sulphur and phosphorus. Thus, ICPMS is a very good technique to detect silica particles (Si) and semiconductor nanocrystals (Cd, Se, Zn, S etc.). Even polymeric particles can be detected with ICPMS, if they are labelled or stained with respective elemental tags or fluorophores containing heavy metal ions like lanthanides. Reference-free quantification in GIXRF analysis is based on knowledge of all experimental parameters due to radiometrically calibrated XRS instrumentation and the employment of atomic fundamental parameters such as interaction cross sections and yields, thus replacing the conventional needs for reference materials or calibration standards. The project will then investigate the ability of laboratory based instrumentation to measure shell chemistry and thickness, including different methods to assess the mean number of accessible functional groups at the surface of particles. This is very important, for example, in biomedical and bioanalytical applications requiring subsequent surface functionalisation.