Calcium Phosphate Phase Identification Using XPS and Time-of-Flight Cluster SIMS

Reproducible time-of-flight cluster static secondary ion mass spectra (ToF-SSIMS) were obtained for various standard calcium phosphate (CP) powders, which allowed for phase identification. X-ray diffraction was not able to detect signals from microscopic amounts of CP (∼15 mmol m-2). The phases studied were α-tricalcium phosphate [α-Ca3(PO4)2], β-tricalcium phosphate [β-Ca3(PO4)2], amorphous calcium phosphate [Ca3(PO4)2·xH2O], octacalcium phosphate [Ca8H2(PO4)6·H2O], brushite (CaHPO4·2H2O), and hydroxyapatite [Ca10(PO4)6(OH)2]. The SIMS spectra were obtained via bombardment with (CsI)Cs+ projectiles. X-ray photoelectron spectroscopy (XPS) core levels of the P 2p, Ca 2p, and O 1s orbitals and the relative O 1s loss intensity were examined. The PO3-/PO2- ratios from ToF-SSIMS spectra in conjunction with XPS of the CP powders showed much promise in differentiating between these phases at microscopic CP coverages on the metal oxide surface.