The presence of oxygen has significant effects in a mixed argon–oxygen glow discharge plasma. Previous studies of these effects did not use added oxygen in concentrations that would be expected in typical analytical oxide samples in glow discharge. We report results of studies using glow discharge optical emission spectrometry (GD-OES) to investigate the effects when oxygen comes from within the sample itself, here Calamine, i.e. an iron sample covered by an oxide layer, in a pure argon plasma. Time-resolved spectrochemical information acquired during the analysis of the oxide layer is discussed, and GD-OES depth profile measurements with this sample, obtained by sputtering in pure argon, are also presented. Results are compared with those observed with controlled addition of oxygen to the main carrier gas (argon). It is concluded that analytically GD-OES results obtained from the FeO_x (Calamine) sample are approximately equal to a pure iron sample sputtered in argon with 0.05% v/v oxygen concentration. The linear dependence of reduced sputter rate, measured by using a Calamine layer of known thickness, on voltage thus recorded is seen as apparent confirmation of the Boumans equation. However, in the case of a pure iron sample with various argon–oxygen mixtures, the results are different from those obtained with Calamine as a sample.