Micellar and aqueous-organic liquid chromatography using sub-2 μm packings for fast separation of natural phenolic compounds

Interpretive search of optimal isocratic and gradient separations in micellar liquid chromatography in extended organic solvent domains

Micellar liquid chromatography (MLC) is a reversed-phase mode with mobile phases containing an organic solvent and a micellised surfactant. Most procedures developed in MLC are implemented in the isocratic mode, since the general elution problem in chromatography is less troublesome. However, gradient elution may be still useful in MLC to analyse mixtures of compounds within a wide range of polarities, in shorter times. MLC using gradients is attractive to determine by direct injection moderate to low polar compounds in physiological samples. In these analyses, the use of initial micellar conditions (isocratic or gradient) with a fixed amount of surfactant above the critical micellar concentration, keeping the organic solvent content low, will provide better protection of the column against the precipitation of the proteins in the physiological fluid. Once the proteins are swept away, the elution strength can be increased using a positive gradient of organic solvent to reduce the analysis time. This may give rise to the transition from the micellar to the submicellar mode, since micelles are destroyed at sufficiently high concentration of organic solvent. In this work, several retention models covering extended solvent domains in MLC are developed and tested, and applied to investigate the performance in isocratic, linear and multi-linear gradient separations. The study was applied to the screening of β-adrenoceptor antagonists in urine samples, using mobile phases prepared with sodium dodecyl sulphate and 1-propanol. Predicted chromatograms were highly accurate in all situations, although suffered of baseline problems and minor shifts for peaks eluting close to a steep gradient segment. Two columns (C18 and C8) were investigated, with the C8 column being preferable owing to the smaller amount of adsorbed surfactant.

Comparison of Phenolic Compound Separations by HPTLC and PPEC with SDS as the Mobile Phase Component

The application of the surfactant (sodium dodecyl sulphate, SDS) as the component of the water-organic mobile phase in thin-layer chromatography and pressurized planar electrochromatography is presented. The influence of various variables on the separation of various phenolic compounds (flavonoids and phenolic acids) as model compounds with systems containing surfactant is discussed. The effect of concentration of butanol and SDS as well as pH of the mobile phase buffer on migration distance of the solute zones is investigated. The presence of SDS in the eluent affects the butanol solubility in the mobile phase. It allows using higher organic solvent concentration systems compared with the mode without surfactant. The amount of SDS in the eluent has the effect on the solute retention, whereas the eluent buffer pH affects the migration distances of ionisable phenolic acids both in HPTLC and PPEC. The migration distances of flavonoid glycosides are considerably longer than those of pure flavonoids. Considering second group of investigated solutes, derivatives of the benzoic acid migrate longer distances in comparison with the cinnamic acid ones. In addition, in the majority of experiments, ionisable compounds (phenolic acids) migrate longer distances in PPEC than nonionisable compounds (flavonoids). Additionally, the order of solutes differs in the PPEC and HPTLC system.