Ionic liquids versus triethylamine as mobile phase additives in the analysis of β-blockers

A comparative study of peak shape, elution behavior, elution strength and resolution of seven β-blockers (acebutolol, alprenolol, labetalol, metoprolol, nadolol, pindolol and propranolol) chromatographed with aqueous-organic mobile phases containing additives such as the ionic liquid (IL) 1-butyl-3-methylimidazolium (BMIM BF4) or triethylamine (TEA) is performed using a conventional reversed-phase Kromasil C18 column and isocratic elution. The efficiencies and asymmetry factors achieved for the group of β-blockers in the Kromasil C18 column improve when the cationic modifiers are added to the aqueous-organic mobile phase as competing additives for the silanol active sites. BMIM BF4 is a significantly better additive compared to TEA. The improvement is more notably for the asymmetry factor, BMIM BF4 allowing to obtain higher plate numbers than TEA at the same concentration. The effects of both modifiers on elution strength and retention factors are, however, different. TEA decreases the solute retention factors when BMIM BF4 does not change them significantly. Using other additives taken in the IL family such as 1-butyl-3-methylimidazolium hexafluorophosphate and 1-octyl-3-methylimidazolium tetrafluoroborate (OMIM BF4), it is shown that the silanol screening effect is always observed, due to the IL cation, when it is possible to increase or to decrease the solute retention factors playing with the hydrophobic nature or chaotropic character of its anion.