Figure shows experimental obtained during light-induced O2 evolution by a Co/poly(heptazine imide) photocatalyst with gas phase O2 detection. The starting concentration of oxygen is ca. 0.032 vol% and there is a slight increase of oxygen concentration in the pre-reaction phase. After the start of the irradiation, there is an induction period, due to the relatively slow diffusion of O2 from the liquid to the gas phase. Afterwards, the O2 concentration increase roughly linearly. In the post-reaction phase, the O2 concentration continuous to increase, although the increase slows down gradually. This oxygen concentration increase in the post-reaction phase is also due to continued diffusion of photocatalytically formed oxygen from the liquid to the gas phase. To analyze the obtained data, a linear function is fitted to the datapoints of the pre-reaction phase, yielding the baseline for this experiment (see Figure ). The baseline is then subtracted from the experimental data, giving the baseline corrected data shown in Figure . Based on the baseline corrected data we can conclude that 0.022 vol% O2 were formed during the photocatalytic experiment.