Oxygen exchange and diffusion measurements: The importance of extracting the correct initial and boundary conditions

18O / 16O exchange followed by Secondary Ion Mass Spectrometry (SIMS) analysis is often used to determine the oxygen tracer diffusion coefficient D* and the oxygen surface exchange coefficient k* of oxide materials. Conventionally, the experiment consists of annealing an equilibrated, semi-infinite medium, in which the initial isotope fraction is cbg, in a volume of gas of constant isotope fraction cg. In this paper we consider the errors that may be introduced into the transport parameters if the conventional initial and boundary conditions are assumed, when extracting D* and k* from experimental data, but the actual conditions, under which the anneal was carried out, are significantly different. Particular attention is given to the problem of isotope depletion from the gas phase when annealing materials that display high values of D* and k*.