This Recommendation sets out the requirements for pure gases (CO, CO2, CH4, H2, O2, N2 and Ar) used as components for the preparation of reference gas mixtures, in the 0.01 - 0.99 mole ratio range(*). The purpose of this Recommendation is to establish a harmonized method for the elaboration of specifications for those pure gases, which are intended for metro-logical purposes. Technical requirements for pure gases 1. Industrially produced pure gases, supplied under pressure in cylinders, shall be used as reference gases. 2. The mole ratio of the pure gas shall be not less than 0.999 9. 3. The error limit on the evaluation of the mole ratio of the pure gas shall not exceed 1?10- 4. 4. The mole ratio of any impurity (He, Ne, H2, Ar, O2, N2, Kr, CO, CO2, N2O, Xe) shall not exceed 1?10-5 (**). 5. The mole ratio of any corrosive gas (NH3, NmOn, SO2, H2S, COS and others) shall not exceed 1?10-6. 6. The mole ratio of the sum of hydrocarbons shall not exceed 1?10-6. 7. The mole ratio of water vapour shall not exceed 2?10-6.