Secondary ion mass spectrometry is an accepted method of determining the near-surface composition of glass, but the interpretation of the results is not straight-forward. The ionic structure of glass means that the mobile species are often ions, subject to diffusion down a concentration gradient but also driven by the voltage gradient which develops to maintain charge balance. Further the diffusion may rely on transient mobile species rather than direct movement of the predominant ionic species. SIMS profiles obtained from glass samples held at high relative humidity and at temperatures such that hydrated soda rather than a liquid phase exists at the surface prove to be extremely complex, with both soda and water apparently diffusing against their concentration gradients over parts of the depth profile. It is shown that there is enough independent information in these different parts to indicate the dominant mobile species and to provide a lot of information about their diffusivity and also their mobility under a voltage gradient. In particular the data appears to confirm previous speculations that H2O is an important mobile species in this regime.
