Natural abundance 43Ca solid-state NMR characterisation of hydroxyapatite: identification of the two calcium sites

Natural abundance (43)Ca solid-state NMR of hydroxyapatite (Ca(10)(PO(4))(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 (43)Ca NMR interaction parameters (delta(iso), C(Q) and eta(Q)) of each site were determined by multiple magnetic-field simulations. The peaks with delta(iso) = 11.2 +/- 0.8 and - 1.8 +/- 0.8 ppm, both with C(Q) = 2.6 +/- 0.4 MHz, were assigned to the Ca(II) and Ca(I) sites, respectively, on the basis of their relative intensities.