Fig. 2. ³¹Phosphorus NMR spectra (2.0 kHz spin frequency) of: (a) pure-, Cu12-, Cu20-, Fe12-, Fe25-, Mn11-, and Mn24-synthetic hydroxyapatite (SHA); and (b) pure-SHA and pure-SHA mixed with 1.0 wt.% of MnSO₄·H₂O (Mn-mix), ferrihydrite (5Fe₂O₃·9H₂O) (Fe-mix), Cu(NO₃)₂·2.5H₂O (Cu-mix), and MgSO₄ (Mg-mix). Spectral intensities were scaled for ease of comparison.