Surface characterization of hydroxyapatite and related calcium phosphates by XPS and TOF-SIMS

The surfaces of six biologically interesting calcium phosphate (CaP) phases (hydroxyapatite, dibasic calcium phosphate dihydrate, dibasic calcium phos phate, monobasic calcium phosphate, beta-tribasic calcium phosphate, octaca lcium phosphate) have been examined by X-ray photoelectron spectroscopy (XP S) and time-of-flight secondary ion mass spectrometry (TOF-SIMS), The inten sity of an O(1s) shake-up satellite correlates with the phosphate oxygen co ntent. Together with the Ca/P and O/Ca XPS peak ratios, this feature helps provide identification of the CaP phase(s) present in the surface of unknow n samples and establish their mole fractions, as proven with a bone sample. Contributions from carbonate impurities can be quantified using its C(1s) peak at 279.9 eV and subtracted from the O(1s) line shape to aid identifica tion. Principal component analysis (PCA) has been applied successfully to a nalyze TOF-SIMS spectra of these six CaP phases. Multivariate analysis can help differentiate these CaP phases using the first two PCs, which are domi nated by the relative intensities of only a few key ions: PO3-, O-, Ca+, Ca OH+, PO2-, and OH-.