Engineered nanoparticles (ENPs) show new and interesting properties leading to an increased use in various application fields and have entered our daily environment (e.g., functionalized clothing, cosmetics, food, medicine). Though on the one hand nanotechnology plays a substantial role in societies' daily life, on the other the presence and behavior of ENPs in organisms and the environment is still unclear to a large extent. Furthermore, comprehensive legislative regulation is still missing. For adequate regulation a clear definition of ENPs is needed. A definition recommendation was released in 2011 by the European Commission (EC) on the basis of size and number of ENPs present within a defined size range. However, straightforward analytical techniques which easily provide information allowing for a decision (based on the EC definition) on the presence and concentration of ENPs in a given sample do not exist yet. A promising tool, offering fraction-related size information on the one hand and allowing for element-specific detection on the other is the coupling of asymmetric flow-field-flow-fractionation (AF4) with inductively coupled plasma-mass spectrometry (ICP-MS). In this work, a new strategy for quantifying silver nanoparticle (AgNP) size fractions (30 nm ± 2.1 nm, 75 nm ± 3.9 nm) after base-line AF4 separation relying on on-line ICP-MS detection combined with “post-channel” species-unspecific on-line isotope dilution (on-line ID) was successfully developed. A limit of detection (LOD) of 0.5 μg Ag L−1 and a limit of quantification (LOQ) of 1.6 μg Ag L−1 were achieved by the approach applied. The recovery values for the smaller size-fraction (30 nm) were in the range of 31–41% while for the larger size-fraction (75 nm) in the range of 75–78%. The overall reproducibility (RSDs, peak areas) was in between 3.4–5.4%. Validation of the on-line ID approach was achieved via off-line fraction collection and total silver determination afterwards; a bias of 2.9–16.4% between both approaches was observed indicating that the on-line ID approach is working properly. To the best of the authors' knowledge, this is the first time that species-unspecific (post-channel) on-line ID was combined with AF4/ICP-SF-MS for fraction-related quantification of AgNPs.
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