“…Thanks to a number of international efforts in recent years, this situation is improving. [2][3][4][5][6][7][8][9][10][11][12] Documentary standards have been developed within the International Organization for Standardisation (ISO) for a number of techniques capable of performing number concentration measurements including population-averaging methods such as: small angle X-ray scattering (SAXS), 13 dynamic light scattering 14,15 and centrifugal liquid sedimentation (CLS), 16,17 as well as particle-counting methods such as, single particle inductively coupled plasma mass spectrometry (spICP-MS), 18 particle tracking analysis (PTA), 19 differential mobility analysis with integrated condensation particle counter 20 and resistive pulse sensing (RPS). 21 Many of these standards, however, describe aspects of particle size analysis rather than measurement of number concentration.…”