Surface spectroscopy of apatitic materials: limitations and concerns

The use of x-ray photoelectron spectroscopy for the study of biological materials has provided mixed results. The experimental difficulties associated with the application of this technique to the study of biological materials include the effects of ultra-high vacuum, x-ray exposure, surface charging, and, for insulating materials, the inherent lack of an intrinsic energy reference. Since most biological materials are insulators, the lack of an intrinsic energy reference complicates one's ability to draw direct comparisons between different studies. In this paper, the results from two different apatites are compared with the results obtained from the use of two conventional techniques for referencing binding energies. The results presented here are referenced only to the Pt Fermi level; no further energy modifications are performed. A comparison with the results of other investigators indicates a large difference in binding energies for all involved core levels. These results indicate that, in the study of insulating materials with x-ray photoelectron spectroscopy, it is essential that all data be acquired under exacting conditions on the same experimental system.