Potential of proton microbeam in the study of dental composites. 2. Compositional heterogeneity by micro-PIXE and complementary analysis by micro-PIGE and micro-PBS

To investigate the structural and chemical heterogeneity of dental composites, we analyzed their composition both in whole maps and in selected points by μPIXE, μPIGE and μPBS (Proton Backscattering). Two qualitatively similar biomaterials labeled IV.a and V were studied. Thick disk-shaped, flat-surface samples were prepared by photopolymerization on a glass plate, and investigated at the Rossendorf nuclear microprobe with a 3.1 MeV proton beam focused to a ~3 μm spot. Areas of 250 x 250 μm² were mapped, and selected inclusions were analyzed in point mode using three detectors simultaneously. Five areas located ~0.5-2 mm apart of each other and five inclusions from a map for IV.a, as well as one area for V were analysed. Hydroxylapatite and pyrite were used as reference materials. Concentrations from μPIXE spectra were determined by GUPIX calculations. Up to 19 elements were detected, i.e. Ca, Zr, Ba, Yb (major), Fe, Sr, Hf (minor), Mn, Se, Ho (traces) by μPIXE; F, Na, Al, Si by μPIGE; and C, O, F, Al, Si, Ca (and possibly N, Na, Cl) by μPBS. As compared to IV.a, in composite V higher Ca, lower Zr and Ba, and similar Yb levels were found. The mixture in composite IV.a appeared as a heterogeneous map both on a ~1.0 and on a ~0.1 mm scale of the sample. Thus in all μPIXE maps the absolute concentrations of Ca, Zr, Ba and Yb showed rather high dispersion (relative standard deviations ~31-44 %), but the ratios Zr/Ca, Ba/Ca and Yb/Ca were largely constant. The inclusions analyzed in point mode were Zr-rich and although their relative concentrations were roughly constant the absolute ones showed very high dispersion (rel. SD ~120 %). Hence proton microprobe analysis methods are well suited to study the dental composites and to characterize in detail their heterogeneous structure.