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We have shown that the amount fraction of carbon dioxide in a nitrogen or synthetic air matrix stored in cylinders increases as the pressure of the gas mixture reduces, while the amount fraction of methane remains unchanged. Our measurements show the initial amount fraction of carbon dioxide to be lower than the gravimetric value after preparation, which we attribute to the adsorption of a proportion of the molecules to active sites on the internal surface of the cylinder and the valve. As the mixture is consumed, the pressure in the cylinder reduces and the amount fraction of the component is observed to increase. The effect is less pronounced in the presence of water vapor. More dramatic effects have been observed for hydrogen chloride. These findings have significant implications for the preparation of high accuracy gaseous reference materials with unprecedented uncertainties which underpin a broad range of requirements, in particular atmospheric monitoring of high impact greenhouse gases. more...