X-ray-induced photoelectron spectroscopy of dentin as a function of duration of Ar(+)-ion etching

Laboratory for Materia Technica, School of Medical Sciences, University of Groningen, The Netherlands.

XPS, or ESCA, measures the atomic concentration in the outermost layers of the sample. The surface is often etched with Ar(+)-ions before any measurement. We investigated the role thereof on XPS measurements of root dentin. Nine slices were cut from premolars. Slices were ground or broken. XPS was measured before etching. Six samples were then subjected to Ar(+)-ion etching (10 kV ions, 0.25 microA/mm2). A gold sample was also included. Relative concentrations of N and C decreased with etching time, while O, P, and Ca increased. N and C curves were analyzed assuming a sum of two exponential decays and a final level. On average, tau 1 = 20 sec for C and 17 sec for N. On average, tau 2 = 278 sec for C and 350 sec for N. No differences between differently prepared samples were apparent. The gold sample showed a single decay to noise for C and N, with tau 1 = 8 and 7 sec, respectively. We conclude that two decay processes are present, due to gas removal and to the removal of organic material from the dentin matrix, respectively. Thus, true values can be obtained by extrapolation to t = 0 of only the data obtained by summing the slow decay and the final level.