Nonlinear Distribution Coefficients in Gas Chromatography

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Abstract
Nonlinear partition coefficients of polar solutes on coated and uncoated diatomaceous-earth supports lend themselves to mathematical treatment when proper precautions are taken to reproducibly condition the chromatographic column. A previously derived equation was used to calculate the adsorption and distribution isotherms from adsorption-desorption chromatograms whose diffuse elution curves approached the base line asymptotically. In the concentration ranges studied, the adsorption of acetone on uncoated support obeyed the Temkin isotherm equation. A plot of the amount adsorbed vs. the log of the concentration of the solute in the carrier gas gave a straight line. The adsorption of acetone on coated columns under the same conditions followed a Freundlich isotherm equation, and log-log plots of amount adsorbed vs. concentration of the solute gave straight lines. The equations for the lines obtained from the isotherm plots of the coated and uncoated columns were used to calculate the retention volumes as a function of concentration, by means of the equation VNg = dQ/dC. The validity of the retention volume and adsorption equations were checked by the agreement of theoretical and experimental measurements. Within limits retention volumes may be correlated with sample size, providing the contribution of the bulk liquid phase is known. It is expected that the retention volume-sample size relationship would be more useful in low-loaded columns, where the bulk liquid-phase contribution is either small or negligible. The temperature-effect studies showed that, as expected, stronger adsorption bonds are formed at lower temperatures. The sample size-temperature studies on the inactive (RIC) support coated with tri-o-tolyl phosphate showed that the liquid phase contributed to the non-linearity of the distribution coefficient in a minor but unknown extent.