A summary of D-SIMS analysis results is shown Figure , with raw data shown in Figure . Each bar depicts the mean and standard deviation of 3 depth profiles, each measured over 120 s of sputtering. ZnSe(ba) treated with non-isotopically enriched H2O and O2 exhibited a O/ O ratio nearly identical to the natural isotopic abundance ratio of 0.204%, confirming that the ratio of the mass measurements corresponds to the ratio of O isotope present in the material. When isotopically labelled O2 was used in place of naturally abundant O2, the O/ O ratio increased by a factor of 4-5 times to 0.9%. This result supports the hypothesis that oxygen atoms from O2 are part of the ZnO degradation product.