pH characterization of methanol/water/carbon dioxide mixtures: Fundamentals and applications in chromatography

Enhanced-fluidity liquids are mixtures of common liquids and liquefied gas that are compressed into one phase. These mixtures often include H 2O as one of the components to maintain sufficient polarity. If combined with CO2, the pH of these mixtures will change due to the formation of carbonic acid. The impact of CO2 on the acidity of enhanced-fluidity liquids can be considered as two fold. First, the formation of carbonic acid will acidify the mixture. Secondly, the addition of nonpolar CO2 will result in a mixture with a lower dielectric constant, which influences the weak acid-base equilibria in the mixture. A spectrophotometric means to measure pH in methanol/H2O/CO2 mixtures is illustrated. From the UV/Vis absorption spectra of several pH indicators, pH values of four methanol/H2O/CO2 mixtures at room temperature are determined. pH control in enhanced-fluidity liquids is demonstrated by replacing water with aqueous buffers to alter the pH value. Meanwhile, the impact of pressure on the acidity of methanol/H2O/CO2 mixtures is found to be minimal.; Separations of several groups of ionogenic disubstituted benzene isomers are achieved by taking advantage of the pH variation caused by different CO 2 content in the methanol/H2O/CO2 mobile phases without the addition of aqueous buffer. The correlation between mobile phase pH and dissociation constant of the analytes successfully explains the retention behavior of acidic, basic, and zwitterionic isomers. Separations are evaluated in terms of retention factor (k), separation factor (alpha), peak asymmetry (Asy10), and efficiency (n). Comparison with reversed-phase high performance liquid chromatography (HPLC) of these isomers in literature reveals that resolution improvement for anisic acid, toluic acid and aminobenzoic acid isomers is achieved with methanol/H2O/CO2 enhanced-fluidity liquids. Significant reduction in retention factor is obtained. Peak asymmetry and separation efficiency also are generally improved with the amount of CO 2 in the mobile phases. Meanwhile, the detrimental effect of aqueous buffer to the stationary phase is avoided.; The impact of CO2 concentration in methanol/aqueous solution/CO 2 mobile phases on separation is explored by studying the correlation of retention of several substituted benzoic acids and mobile phase pH in these mixtures. As the pH of these acids increases with the amount of CO2 in these mixtures, more basic aqueous buffer is needed to control the dissociation of these acids in order to obtain better separation. Variation of separation efficiency and peak asymmetry is also related to dissociation of these acids. Adding aqueous buffer into methanol/H2O/CO2 is an effective means to optimize separations of acidic analytes with enhanced-fluidity liquids.