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This International Standard defines the following quantities commonly used to express the composition of gas mixtures: -- mole fraction, -- mass fraction, -- volume fraction, and -- mole concentration, -- mass concentration, -- volume concentration. For these quantities of composition, this International Standard provides methods for -- conversion between different quantities, and -- conversion between different state conditions. Conversion between different quantities means calculating the numerical value of an analyte content in terms of one of the quantities listed above from the numerical value of the same analyte content, at the same pressure and temperature of the gas mixture, given in terms of another of these quantities. Conversion between different state conditions means calculating the numerical value of an analyte content, in terms of one of the quantities listed above, under one set of state conditions from the numerical value of the same quantity under another set of state conditions, i.e. pressure and temperature, of the gas mixture. Gas mixture composition can be converted simultaneously between different quantities of composition and different state conditions by combination of the two types of conversion. This International Standard is applicable only to homogeneous and stable gas mixtures. Therefore any state conditions (pressure and temperature) considered need to be well outside the condensation region of the gas mixture and that of each of the specified analytes (see Annex A).