ABSTRACT
In this study, water was used as an additive in the methanol-modified carbon dioxide-based eluent for the elution of some basic organic compounds from a hybrid silica column via supercritical fluid chromatography (SFC). The experiments were applied to sulfonamides, propranolol, and other organic nitrogen compounds involving aromatic rings from different classes of amine, pyrimidine, and purine with different pKa values (the pKa values for the studied analytes range from 4.6 to 10.4). The results revealed different responses to the different percentages of water addition. Adding 1~2% of water to the modifier (methanol) led to a positive effect manifested by more symmetrical peak shapes and reduced retention times for most compounds. The key factor for this improvement in the properties of chromatographic peaks is due to the adsorption of water on the silanol groups of the stationary phase, consequently resembling the phenomena observed in hydrophilic interaction liquid chromatography (HILIC). Moreover, the availability of hydrogen bond acceptor and donor sites in the analyte structure is an important factor to be considered when adding water as an additive to the modifier for improving the chromatographic peaks. However, introducing water in an amount higher than 3% resulted in perturbed chromatographic signals. It was also found that water as an additive alone could not successfully elute propranolol from the hybrid silica column with an acceptable peak shape; thus, the addition of a strong base such as amine salts was also necessary. The proposed use of a particular amount of water in the mobile phase could have a positive effect compared to the same mobile phase without water, improving the chromatographic peak properties of the elution of some basic organic compounds from the hybrid silica column.
ABSTRACT
The surface excess isotherms of methanol from carbon dioxide on reversed phase stationary phases under two different operational conditions - which can be considered subcritical and supercritical conditions depending on the molar fraction of CO2 in methanol - were determined using the minor disturbance peak method. The shapes of the surface excess isotherms were very similar in subcritical and supercritical conditions for the same column. To verify the influence of the sample solvent on the separation efficiency, two solvents methanol and heptane were used as sample solvents for alkylbenzene samples for the separation on the studied columns with pure carbon dioxide mobile phase. The separation efficiency was determined by calculating the number of theoretical plates. On the embedded amide stationary phase with methanol as a sample solvent the efficiency has increased due to the displacement effect of methanol on the solutes which are retained less than methanol. Then the efficiency for the rest of solutes, which coincide with the elution of the methanol peak tail has decreased as a result of the tag-along effect. The surface adsorbent heterogeneity has been discussed; the bonded ligands on the stationary phase surface demonstrated adsorption a big amount of CO2, while methanol could adsorb with small amount on the residual silanols on the surface of stationary phase and the embedded (amide) polar group in the bonded phase.
