Determination of Gas-Liquid and Gas-Solid Equilibrium Isotherms by Chromatography: I. Theory of the Step-and-Pulse Method

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Abstract
Theoretical difficulties to relate the signals obtained to the isotherm have prevented the use of chromatographic methods in the determination of partition or adsorption isotherms at large concentrations. The use of a general model of propagation of chromatographic signals indicates the exact origin of these difficulties. This model takes into account the sorption effect (influence of the finite concentration of vapors on the migration velocity of the gas phase), the isotherm effect, and the pressure gradient. It neglects the effects of diffusion and resistance to mass transfer but can be used to solve exactly the problem of the step-and-pulse method. In this method analytical chromatography is carried out with a gas phase made of a mixture of an inert gas and the vapors of the solutes under study, at a constant concentration, which can be set in successive analysis at different values in the concentration range investigated. The equations relating the retention time of the signals generated by a pulse, injected over this step of constant concentration, are derived, and it is shown how this method can be used to determine isotherms and other physical parameters.

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