Performance of amines as silanol suppressors in reversed-phase liquid chromatography

In reversed-phase liquid chromatography, cationic basic compounds yield broad and asymmetrical peaks, as a result of their ionic interaction with the anionic free silanol groups present in the silica-based stationary phases (commonly derivatised with C18 groups). A simple way to improve the peak shape is the addition to the hydro-organic mobile phase of a reagent (usually called additive) with cationic character. This associates with the stationary phase to prevent the access of analytes to the free silanol groups. Cationic additives may interact electrostatically with the anionic silanols. The hydrophobic region of the additive may also associate with the alkyl chains bound to the stationary phase, with the positive charge oriented towards the mobile phase. The access to the silanol groups is thus blocked, but in turn, the stationary phase is positively charged and will repel the protonated basic compounds, which unless their polarity is sufficiently low, will elute at very short times. In this work, a comparative study of the performance of a group of amines (butylamine, pentylamine, hexylamine, cyclopentylamine, cycloheptylamine, N,N-dimethyloctylamine and tributylmethylammonium chloride), as modifiers of the chromatographic behaviour of basic compounds, is carried out. The behaviour is compared with that obtained with the ionic liquids 1-butyl-3-methylimidazolium chloride and 1-hexyl-3-methylimidazolium chloride, used as additives. The study revealed that the performance of the cationic additives to block the silanol activity is mainly explained by the additive size and its ability to be adsorbed onto the stationary phase.