Retention Characteristics of a pH Tunable Water Stationary Phase in Supercritical Fluid Chromatography

The retention characteristics of a novel pH tunable water stationary phase are presented. The method utilizes a change in mobile phase from N₂ to CO₂ to acidify the water phase in situ and control the ionization and elution of organic acids. With N₂ present the phase pH > 5.4 and the acids are ionized and strongly retained. Conversely, with CO₂ present the pH < 3.8 and the acids are neutralized and can elute. This effect is reasonably independent of time. For example, at 80°C hexanoic acid readily elutes from a 10 m column after switching to CO₂ at any point over a 1 h period. Beyond this, however, some broadening and peak erosion is noted. Acids are also retained on 10 and 2 m columns similarly, since their elution primarily depends upon the change in stationary phase pH. Altering the CO₂ solubility in the water phase alone (i.e., through changing system temperature and pressure without using N₂) also produces similar changes in stationary phase acidity. However, this approach yields greater system noise and instability. The N₂/CO₂ switching mode is used to analyze organic acids in various samples and is found to provide high selectivity for them over other matrix components. Therefore, this approach can potentially simplify the analysis of such acids in complex samples.