Thus, 400 mM phosphate-containing solutions at different pH were cryo-fixed using the setup described above and analyzed at low temperature by P MAS ssNMR at 120°C. The resulting P isotropic resonances (Figure ) differ significantly from solution-state NMR spectra. They are 100 times broader (average LWHM of 4.5 ppm) and display a Gaussian line-shape. Importantly, no reprecipitation of the initial phosphate salt or crystallization of a phosphate phase is observed. These observations support the efficient cryofixation of the solutions and the dispersion of the phosphates in a vitreous matrix. However, the distinct contribution of individual P resonance of the different phosphate species is not observed contrary to carbonate solutions where the distinct contributions of HCO3 -and CO3 2-resonances are seen in the low-temperature (100K) C NMR spectrum of a vitrified matrix. Indeed, the contributions from more or less protonated phosphate species (H3PO4, H2PO4 -, HPO4 2-and PO4 3-) are spread over a smaller range compared to HCO3 -and CO3 2-(5 vs 15 ppm). We also observe an increase in iso with increasing pH, similar to liquid NMR, with a marked transition around pH 7.4 and 12 corresponding to the expected pKa2 and pKa3 of phosphate (Figure ). However, contrary to what is anticipated, iso stays constant around pH 2 (pKa1). At slow MAS spinning (MAS = 2 kHz), the resulting spectra (Figure ) differ significantly from the solution-state NMR spectra, as isotropic resonances (orange dashed line in Figure ) are flanked by spinning side bands (SSB). The relative intensities of the SSB manifolds vary according to the pH value. Indeed, for phosphates, the SSB manifold is mainly determined by the CSA, which depends on the local symmetry around the 31 P that is changing with phosphate protonation. Thus, phosphate speciation determines CSA parameters and SSB manifold line shapes.