Identification and elemental mapping of enamel minerals with electron energy-loss spectroscopy

The identification and differentiation of the mineral phases, hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) and octacalcium phosphate (OCP, Ca₈H₂(PO₄)₆), remains challenging because of their similar composition and chemical structure. In this research, electron energy-loss spectroscopy (EELS) analyses revealed indicators to distinguish HA and OCP and these were applied to examine mineral development in enamel from mouse incisors. Reference EELS data for HA and OCP was established with commercial HA and synthesized OCP. An evaluation of electron damage and a mitigation strategy of multipass imaging was conducted, and the electron dose limitation of OCP was identified. New insights into the mechanism of electron beam damage on the apatite crystal were obtained. With the characterization of the energy-loss spectra and the EELS simulation, the oxygen K-edge was found to be one of the indicators for the differentiation of HA from OCP. The second indicator, the Ca/P ratio, was calculated with a calibrated experimental factor of K _exp. Elemental mapping was done to establish the different Ca/P ratio of HA and OCP, and the boundary between these mineral forms. EELS analysis was performed on developing enamel in wild-type (WT) and Mmp20 knockout (KO) mice. This research establishes a protocol for EELS analysis on biological specimens and demonstrates the power and potential of EELS in biomaterial characterization.