A new method was developed for the preparation of highly monodisperse isotopically enriched Si-29 silica nanoparticles (Si-29-silica NPs) with the purpose of using them as spikes for isotope dilution mass spectrometry (IDMS) quantification of silica NPs with natural isotopic distribution. Si-29 tetraethyl orthosilicate (Si-29-TEOS), the silica precursor was prepared in two steps starting from elementary silicon-29 pellets. In the first step Si-29 silicon tetrachloride ((SiCl4)-Si-29) was prepared by heating elementary silicon-29 in chlorine gas stream. By using a multistep cooling system and the dilution of the volatile and moisture-sensitive (SiCl4)-Si-29 in carbon tetrachloride as inert medium we managed to reduce product loss caused by evaporation. Si-29-TEOS was obtained by treating (SiCl4)-Si-29 with absolute ethanol. Structural characterisation of Si-29-TEOS was performed by using H-1 and C-13 nuclear magnetic resonance (NMR) spectroscopy and Fourier-transform infrared (FTIR) spectroscopy. For the NP preparation, a basic amino acid catalysis route was used and the resulting NPs were analysed using transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), dynamic light scattering (DLS) and zeta potential measurements. Finally, the feasibility of using enriched NPs for on-line field-flow fractionation coupled with multi-angle light scattering and inductively coupled plasma mass spectrometry (FFF/MALS/ICP-MS) has been demonstrated. (C) 2015 Elsevier Inc. All rights reserved.