Gas chromatography at finite concentrations. Part 2.—A generalised retention theory

Abstract

The retention theory of gas chromatography is extended to finite concentrations of solute. The detailed treatment takes account of gas compressibility, gas imperfection and variation in velocity of the mobile gas phase due to flux of solute molecules across the interphase boundary. The compressibility-corrected retention volume V^∘_R of a chromatographic zone of given gas phase concentration c is shown to be related to the gradient (∂q/∂c)_p of the partition isotherm by the equation, V^∘_R=V^∘_M+V₁(1 –ajy₀)(∂q/∂c)_p. V^∘_M is the compressibility-corrected gas hold-up; V₁ is the volume of solvent component in the stationary phase in gas-liquid chromatography; P is a defined mean column pressure; y₀ is the mole fraction of solute in the gas phase at the column outlet, corresponding to concentration c; and a and j represent expressions which, for a gas phase showing only moderate deviation from the perfect gas law, are closely approximated by unity and the conventional James-Martin compressibility factor, respectively. A similar equation holds for gas-solid chromatography. The equation becomes inaccurate at high solute concentrations if gas imperfection is large; estimates of the range of conditions in which the equation is valid are made.