Open AccessTechnical Note: An X-ray absorption method for the identification of calcium phosphate species using peak-height ratios
Author(s) -
Julian Oxmann
Publication year - 2014
Publication title -
biogeosciences
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 127
eISSN - 1726-4189
pISSN - 1726-4170
DOI - 10.5194/bg-11-2169-2014
Subject(s) - hydroxylapatite , fluorapatite , apatite , octacalcium phosphate , xanes , amorphous calcium phosphate , brushite , phosphate , mineralogy , calcium , calcium carbonate , chemistry , analytical chemistry (journal) , materials science , spectral line , environmental chemistry , biochemistry , physics , organic chemistry , astronomy , enzyme
X-ray absorption near edge structure (XANES) studies on calcium phosphate species (Ca-P) deal with marginal differences among subtle spectral features despite a hitherto missing systematic breakdown of these differences. Related fingerprinting approaches depend therefore on spectral libraries that are not validated against each other, incomplete and scattered among publications. This study compiled a comprehensive spectral library from published reference compound libraries in order to establish more clear-cut criteria for Ca-P determination by distinctive phosphorus K-edge XANES features. A specifically developed normalization method identified diagnostic spectral features within the compiled library, e.g. by uniform calculation of ratios between white-line and secondary peak heights. Post-processing of the spectra (n = 81) verified distinguishability among most but not all phases, which included hydroxylapatite (HAP), poorly crystalline HAP, amorphous HAP, fluorapatite, carbonate fluorapatite (CFAP), carbonate hydroxylapatite, β-tricalcium phosphate, octacalcium phosphate (OCP), brushite, monetite, monocalcium phosphate, amorphous calcium phosphate (ACP), anapaite, herderite, scholzite, messelite, whiteite and P on CaCO3. Particularly, peak height ratios significantly improved analyte specificity, e.g. by supplementary breakdown into OCP and ACP. The spectral analysis also revealed Ca-P standards that were rarely investigated or inappropriately synthesized, and thus provides a basis for standard selection and synthesis. The developed method and resulting breakdown by species were subsequently tested on Ca-P spectra from studies on bone and sediment. The test indicated that bone material likely comprises only poorly crystalline apatite, which implies direct nucleation of apatite in bone. This biological apatite formation is likely opposed to that of sedimentary apatite, which apparently forms by successive crystallization. Application of the method to μXANES spectra of sediment particles indicated authigenic apatite formation by an OCP precursor