Importantly, it has been previously noticed for carbonate solutions that low-temperature CP MAS experiments of vitrified solutions can be considered semi-quantitative since, surprisingly, the relative intensities of CO3 2-and HCO3 -resonances measured by CP MAS were similar, whatever the contact time, to those measured through direct polarization. Thus, CO3 2-and HCO3 -proportions in the vitrified matrix can be evaluated, and a speciation diagram at low temperatures can be drawn from the CP MAS experiment. We checked that assumption for the present study, and a similar behavior was observed for phosphate cryo-fixed At pH 7.4, 31 P direct polarization and CP MAS spectra exhibit identical signals (position and LW, for contact times ranging from 0.5 to 10 ms) despite the presence of two species having two different protonation states (HPO4 2-and H2PO4 -) (Figure ). The relative intensities of HPO4 2-and H2PO4 -at pH 7.4 are identical, regardless of the acquisition sequence used (CP MAS or direct polarization). Thus, P CP MAS experiments can be used to quantify the proportions of species in the cryo-fixed solutions.