AWS D1.1 2010 3.7.4 say you shall use A 5.32 gas. A5.32 says you are allowed +/-10% of the minor component as a range before the gas falls into another classification. The nominal composition is 25% CO2, the min/max is 22.5% to 27.5%. If your gas is outside those ranges it either needs to be considered:
out of spec and not usable for this WPS
or another classification, and labeled as such which would be a change in nominal composition
3.7.4 says that in addition to the above, the gas in the procedure must be either the gas used for A5.XX specification or a gas that is recommended for use by the manufacturer. If the gas is recommended and is not the standard gas used in the A5.XX for that wire, the recommendation shall be supported by test results as required by the applicable A5.XX document