Because size and size distribution play an important role in the environmental fate and impacts of nanomaterials (NMs), accurate determination of particle size distributions (PSDs) remains a priority need. Despite a diverse range of methods to measure PSD, several challenges remain in the forefront. At the most fundamental level, the different ways of presenting the central size value, and the distribution of particles around this value, lead to potential for ambiguity. Number-, area-, mass- and intensity-based distributions will all appear different for particles other than very monodisperse samples. This may lead to the need to interconvert these distributions. Size-dependant analytical limitations of each method may lead to errors in this conversion, especially at the tails of the distributions.The presence of interfering particles in natural samples leads towards the need to develop tools with some degree of specificity that can lead to discrimination between engineered and naturally occurring particle types. Inductively coupled plasma-mass spectrometry-based methods seem to hold significant promise in this regard but further method development is needed.Finally it is important to recognize the inherent instability of NMs in solution that may lead to aggregation and alter the representation of particle size. This process may be at least in part reversible. Thus PSD is not a static number but is likely very dynamic changing in time and space. Since most methods for PSD analysis have not been validated for aggregates, considerable variability in results from different research groups is to be expected. Until the degree to which aggregation state can be maintained through sampling, storage and analysis is determined, rigorous disaggregation and PSD analysis of primary particles will provide the most robust and comparable size data.
