Elution and preliminary structure-retention modeling of polar and ionic substances in supercritical fluid...

Supercritical fluid chromatography has primarily been applied to relatively nonpolar analytes, even when polar organic solvents are used as modifiers. Here, we show that low levels of volatile ammonium salts as mobile-phase additives allow the elution of polar and even ionic organic materials such as sulfonate salts, carboxylate salts, polyamines, and quaternary ammonium salts. Also, volatile ammonium salts are compatible with mass spectrometric detection, in contrast to other common additives. We have performed preliminary structure-activity-relationship modeling for retention in the CO2/methanol/NH4OAc/Deltabond Cyano system. We have developed a three-descriptor model, where one descriptor, the "relative negative charged surface" explains over 61% of the variance in the retention values. We suggest that two mechanisms have the greatest influence on retention in this system. One is related to the presence of a volatile ammonium salt, the other is related to the ability of a molecule to "hide" its atom with the greatest partial negative charge.