SUPERCRITICAL FLUID CHROMATOGRAPHIC RETENTION OF ANILINES WITH PURE AND MODIFIED FLUIDS ON STANDARD AND DEACTIVATED COLUMNS

Anilines and toluidines produce asymmetric peaks on both standard and deactivated columns using pure carbon dioxide, Freon-13, or Freon-23 as the chromatographic mobile phase. When methanol is added to any of the fluids, peaks become symmetrical. Similar peak shapes are obtained with Freon-13 and Freon-23 but selectivity changes compared to carbon dioxide. The relative retention on cyanopropyl, aminopropyl, Diol, and sulfonic acid columns with the three fluids is studied. In several instances, retention actually increases when modifier is added. Similarly, an increase in pressure (and density) sometimes increases retention slightly. Further, conditions are explored which leave a film of adsorbed modifier on the stationary phase after the modifier is removed from the mobile phase. The observations suggest that, besides covering active sites and modifying the mobile phase, the modifier changes the volume and/or the polarity of the stationary phase.