Hydrodynamic chromatography – inductively coupled plasma mass spectrometry (HDC-ICP-MS) is a technique that is widely used in the size-characterisation of nanoparticles. In this work, a system was modified to facilitate the injection of NIST-traceable standards into the post-column effluent, which then allowed the response from an eluting nanoparticle to be quantified against the response from the post-column standards. Combining the simultaneously acquired particle sizing data and mass concentration data allowed accurate quantification of the particle number concentration to be made in a single analytical run. This unique single-method approach was successfully validated against a nanoparticle system which had previously been characterised in a number of recent peer-reviewed publications. In addition to this, its robustness was assessed using extracts from a study investigating the fate of nanoparticles in sewage sludge, and found to provide much improved data compared to what might have been achieved using an external calibration approach. With in-vial limits of detection of 2 and 10 ng ml⁻¹ for titanium and silver respectively, it is insufficient for use with environmental waters, but is foreseen as being useful in screening nanoparticle production processes, or in the characterisation of higher concentration materials. As this instrumental configuration is likely to be of use to researchers involved in the general area of quantitative trace element speciation, detailed description of construction of the interface is given as ESI.