I have had some puzzling results with regards to packaging a product in a carbon dioxide/ nitrogen blend. We are packaging a sealed syringe filled with 65% carbon dioxide / /35% Nitrogen. The syringe is unfilled Polycarbonate and it is packaged in a foil pouch with the same gas mixture. Our goal is to have the contents of the syringe maintain the gas blend over a 2 year shelf life. The trouble is that the poly is absorbing the carbon dioxide and upsetting the gas balance.

We have tried "soaking" the contents in both 100% carbon dioxide and the 65% carbon dioxide or periods of time (up to a couple months) and under pressure, but have found the results are erratic. When we perform shelf life aging (at 45°C) the % carbon dioxide goes down over a couple weeks, then up again over the next couple weeks. Can you give me a better understanding of what is occurring with the gass exchange through the sealed polymer syringe? And can you offer any solutions?