# Characterization of Nanoparticle Batch-To-Batch Variability
## 1 Introduction
The Organization for Economic Co-operation and Development (OECD) set up a working party on manufactured nanomaterials (WPMN) to define and update critical issues concerning the analysis of physico-chemical properties [14; 15]. Available techniques were compared and validated for a distinct and limited set of NMs. Three major categories were suggested:  
* Characterisation (\"what they are\")--physical and chemical identification in terms of composition, impurities, size and size distribution, shape, surface characteristics and so forth;
* Fate (\"where they go\"): biological (toxicokinetics, biodistribution) and environmental fate described by solubility, hydrophobicity, dispersibility, dustiness and so forth and;
* (Re)activity (\"what they do\"): their reactivity, physical hazards, biological reactivity, toxicodynamics, photoreactivity and so forth. Whereas the focus of these studies was primarily on the identification and validation of suitable methods identifying potential pitfalls and pro and cons dependent on the method and NM under investigation, only a restricted number of NMs were investigated. Hence, the aim of this work was to extend this work by analysing more NMs produced by the same synthesis route (B2B variability) or different routes and assess the impact and reproducibility of synthesis procedures on physico-chemical properties. We looked at four commonly employed NM types from the OECD priority particle list [16], namely SiO\({}_{2}\), TiO\({}_{2}\), CeO\({}_{2}\) and ZnO. Amine-modified polystyrene was also assessed, as this material is often used as a positive control in nanotoxicity experiments [17]. Aqueous synthesis and flame synthesis routes were employed and evaluated for B2B variability and sources of variability that potentially have biological implications are discussed. Nanomaterial synthesis processes were maintained in highly controlled reactors with specific parameters (e.g., mixing conditions, temperature etc.) and were varied systematically in each case to look at batch-to-batch variation under a range of conditions. Physiochemical properties of the resultant batches including most of the relevant OECD proposed parameters [14; 18] complemented with surface structural and compositional analysis were measured by a set of techniques as detailed in Table 1 in an effort to characterize both dry and dispersed NM samples to the highest degree using state of the art approaches. Based on these data, physicochemical properties for the most widely used NMs are discussed which are most vulnerable to B2B variability. Likely potential sources of B2B variability and strategies to mitigate them, as well as the impact of B2B variability for NM safety assessment are discussed. In concordance with previous findings mainly by the OECD WPMN, we confirm the suitability of the proposed techniques and methods including potential shortcomings for a wider range of NMs produced by multiple synthesis routes. Moreover, we used the DCF assay as an additional approach to detect radical formation as a proxy for reactivity. Finally, particle size measurements of especially agglomerated NMs by mobility spectrometry (SMPS) provide more reliable data than by dynamic light scattering (DLS).