Reversed-phase liquid chromatography with mixed micellar mobile phases of Brij-35 and sodium dodecyl sulphate: a method for the analysis of basic compounds

Micellar liquid chromatography (MLC) is a reversed-phase liquid chromatographic (RPLC) mode, which uses a surfactant as a modifier, with significant changes in retention and selectivity with regard to the classical RPLC mode that employs mixtures of water and organic solvent. The anionic sodium dodecyl sulphate (SDS) is the most usual surfactant in MLC, but it also requires the addition of an organic solvent to decrease the retention times and increase the efficiency. In particular, positively charged basic compounds are strongly retained by the stationary phase modified by adsorption of SDS monomers and require the addition of a strong solvent, such as propanol or pentanol. The non-ionic surfactant Brij-35 is much less common in MLC, but has the interesting feature of reducing the stationary phase polarity which remains neutral. This decreases the retention significantly and can eliminate the need of an organic solvent, giving rise to successful "green" RPLC procedures. However, the retention of polar compounds may be too short if these do not exhibit specific interactions with the non-ionic surfactant. In this work, MLC with Brij-35 and mixtures of Brij-35 and SDS without an organic solvent are investigated for the analysis of basic compounds. The research has been carried out with tricyclic antidepressants (TCAs) and beta-blockers, which are compounds of pharmaceutical interest with different polarities. The chromatographic performance in the mixed micellar system is examined in terms of retention behaviour and peak profiles, and compared with the performance achieved with MLC systems containing a single surfactant. In the mixed micellar system, the analysis of beta-blockers of diverse polarity is carried out with good resolution and adequate analysis time. For TCAs, mobile phases with only Brij-35 are preferable.