Reproducible high resolution spectra of nonconducting minerals of interest in biological calcification have been obtained by x-ray photoelectron spectroscopy, utilizing a technique we call biased referencing, a combination of gold decoration and electron charge neutralization of the samples. Minerals examined were hydroxyapatite [Ca10(PO4)6(OH)2], fluorapatite [Ca10(PO4)6F2], calcium carbonate (CaCO3), octacalcium phosphate [Ca8H2(PO4)6 ⋅ 5H2O], monetite (CaHPO4), brushite (CaHPO4 ⋅ 2H2O), calcium pyrophosphate (Ca2P2O7), and an amorphous calcium phosphate synthesized at neutral pH (Ca/P=1.47). Samples were pressed into thin wafers onto which a small gold dot was affixed by vacuum deposition. Biased referencing provided calibration of sample core level ionizations (O 1s, Ca 2p, P 2p, F 1s, and C 1s) to the Fermi level of Au 4f7/2 set at a conductive value of 84.0 eV. Measured binding energies represent initial data for these gold-decorated insulators and may be used to compare data similarly generated from mineralized biological tissues.
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Landis et al. (1984) studied this question.