Eco-friendly liquid chromatographic separations based on the use of cyclodextrins as mobile phase additives

Abstract

Acetonitrile and methanol are the most popular solvents employed in analytical HPLC, but they suffer from a number of drawbacks from the environmental point of view. Alternative, greener mobile phases employing methanol or the less toxic solvent ethanol as the sole organic solvent are proposed in this paper, and applied to the problem of the separation of β-carbolines on C₁₈-stationary phases. The use of β-cyclodextrin (β-CD) and (2-hydroxypropyl)-β-cyclodextrin (HPβ-CD) as mobile phase additives allowed us to increase the proportion of water in the mobile phases without loss in the resolution or efficiency of the separations, leading initially to a considerable reduction of the proportion of methanol in the mobile phase (from 70% to 50%) and at a later stage, to the development of a mobile phase containing only 30% of ethanol. The β-carboline–cyclodextrin association constants were determined by HPLC, and the inclusion complexes were also characterized by ¹H-NMR, ¹³C-NMR and 2D-ROESY experiments, and these studies were used to explain the chromatographic behaviour. The new chromatographic methodology developed was validated and applied to the quantitation of β-carboline derivatives in spiked human serum samples. For the extraction of β-carboline alkaloids from serum samples, liquid–liquid extraction (LLE) and solid-phase extraction (SPE) procedures were compared. It was concluded that the combination of a pre-treatment procedure (ionic exchange SPE) with a water-enriched chromatographic separation leads to a promising, environmentally friendly new methodology.