It is possible to create a plastron of pure oxygen by placing beetles without plastrons into an atmosphere of pure oxygen. When beetles with oxygen plastron submerge, the plastron shrinks over the first several days, then stabilizes. A plastron created with air does not. Why?
The air of the plastron is in equilibrium with gasses dissolved in water. In a natural air plastron, the insect uses oxygen from the plastron for respiration depleting the oxygen concentration in the plastron. Oxygen dissolved in the water diffuses into the plastron and maintains oxygen concentration in the plastron at a steady state.
In an oxygen-only plastron, oxygen from the plastron will diffuse into the water. Nitrogen in the water will diffuse into the plastron but oxygen diffusion is more rapid and the plastron shrinks. The insect is also removing oxygen from the plastron. Eventually the plastron reaches a steady state where the concentration of gasses in the plastron balance concentration of gasses in the water.
Plastrons are an interesting adaptation for breathing under water. The plastron must be large enough to support the rate of oxygen consumption of the insect. As plastron size increases, stability decreases. Theoretically, a large enough plastron could support a human underwater. Practically the massive size necessary would reduce mobility.