Natural abundance43Ca solid-state NMR characterisation of hydroxyapatite: identification of the two calcium sites

Natural abundance 43Ca solid-state NMR of hydroxyapatite (Ca10(PO4)6(OH)2) was performed at three different fields (8.45, 14.1 and 18.8 T). The two crystallographically distinct calcium sites of the apatite structure were spectroscopically resolved at 18.8 T. The 43Ca NMR interaction parameters (δiso, CQ and etaQ) of each site were determined by multiple magnetic-field simulations. The peaks with δiso = 11.2 ± 0.8 and − 1.8 ± 0.8 ppm, both with CQ = 2.6 ± 0.4 MHz, were assigned to the Ca(II) and Ca(I) sites, respectively, on the basis of their relative intensities