The present invention relates to methods, apparatus and systems for producing a gas derived from the atmosphere which is enriched in oxygen or nitrogen for use in medicine, aquaculture, and various industrial applications. The invention may be embodied in a method for purifying a target gas from a mixture having a first and second gas and a second gas, providing a first vessel having a material selective for the first gas over the second gas, providing a second vessel having a material selective for the first gas, contacting the gas mixture to the gas retaining material of the first vessel under conditions allowing for the first gas to be retained by the gas retaining material of the first vessel, allowing the second gas to separate from the first gas retained on the gas retaining material of the first vessel, and allowing the first gas to release from the gas retaining material of the first vessel under conditions that prevent or inhibit mixing of released first gas with the released second gas. In another form the invention may be embodied in an apparatus providing a combined vacuum source and pressure source, the apparatus comprising first and second chambers, a first piston sliding within the first chamber and a second piston sliding within the second chamber, the first and second pistons being coupled such that the first and second pistons move in concert.