Critical aspects of sample handling for direct nanoparticle analysis and analytical challenges using asymmetric field flow fractionation in a multi-detector approach

The analysis of engineered nanomaterials (ENMs), especially engineered nanoparticles (ENPs), is a fast growing analytical research field. New trends in plasma spectrometry such as direct single particle inductively coupled plasma mass spectrometry (spICPMS) or the coupling of asymmetric flow field flow fractionation to ICPMS (A4F-ICPMS) allow direct analysis of ENPs by getting not only chemical but also size information simultaneously. However, sample handling of nanoparticles could be challenging and change the ENP properties. For most of the analysing techniques, dilution of ENP samples is needed as minimum sample preparation. The colloidal stability and the agglomeration behaviour depend on the ENP-type, the coating or functionalization agent and on the surrounding media. The stability of charge stabilized ENPs is especially sensitive to changes of pH or ionic strength, and sometimes even to dilution. Although the stability of sterically stabilized ENPs is typically less affected by the above-mentioned factors, agglomeration can still occur in certain environments. Thus, storage, handling and sample preparation are big challenges in ENP analysis. Kinetic studies of different ENPs, representative of typical nanoparticle types and coatings, point out that the behaviour is dependent on various influencing factors pertaining to the chemical environment (pH, ionic strength, dilution). In this study polyvinyl alcohol (Ag@PVA) and citrate (Ag@citrate) stabilized silver nanoparticles, as well as titanium oxide ENPs functionalized with poly-acrylate (TiO2@PA) have been studied. A simple analytical approach using batch dynamic light scattering (DLS) is proposed for a fast assessment of samples containing unknown ENP types or structures. Besides aspects of sample handling, unwanted particle-membrane interactions, which often lead to inappropriate recovery rates in A4F fractionation, are investigated here. These interactions are caused by the electrostatic charges carried by different membrane materials and the resulting interaction with the ENPs' charge. This is critically discussed for membrane materials typical for A4F analysis: polyethersulfone (PES), regenerated cellulose (RC), and polyvinylidene difluoride (PVDF).