The performance of glow discharge time-of-flight mass spectrometry in isotopic differentiation is revealed using the distribution of oxygen isotopes 16O and 18O in barrier-type anodic alumina films as a focus. Anodic alumina films comprising 18O-rich layers of controlled thickness were formed by the appropriate combination of anodising of superpure aluminium in electrolytes enriched with 18O isotopes and of natural abundance of 18O isotopes. Analysis of the elemental depth profiles of selected ionic species, i.e. 16O18O, allowed determination of the locations of the 18O-rich layers and the 18O/16O interface. Copyright © 2010 John Wiley & Sons, Ltd.