Note on the net retention volume in chromatography with a real gas as a carrier

The problem of the calculation and interpretation of the net retention volu me with a real gas as carrier is revisited. The properties of the carrier a re described by a first order virial equation of state, The net retention v olume of a solute is related to the mean flow rate of the carrier, therefor e determination of the mean flow rate of a real gas carrier is reviewed. It is shown that the mean flow rate cannot be calculated from the mean column pressure. With a real gas carrier the local capacity factor depends on the local pressure of the real gas. The basic relationship between the net ret ention volume and the function describing this pressure dependence is also reviewed. Precise formulae as well as practical approxmations are presented for the calculation of the mean flow rate, of the mean column pressure, an d of a representative pressure related to the mean capacity factor.