Considerations on the possibilities and limitations of comprehensive normal phase–reversed phase liquid chromatography (NPLC×RPLC)

Abstract

A comprehensive normal phase system LC-reversed phase LC (NPLC x RPLC) was evaluated for the separation of a pharmaceutical mixture and citrus oil extracts. NPLC was performed on a 25 cm x 1 mm ID x 5 microm dp diol phase. In the second dimension, an RP 18 monolithic column (10 cm L x 4.6 mm ID x 2 microm macropore size) and an octadecyl silicagel-packed column (5 cm L x 4.6 mm ID x 3.5 microm dp) were applied for the analyses of the pharmaceutical sample and the citrus oil extracts, respectively. A two-position/ten-port switching valve was used as interface. Under optimised LC conditions, the high degree of orthogonality between NP and RP resulted in peak capacities of 300 for the pharmaceutical sample and of 450 for the citrus oil extract composed of lemon and orange oil. Despite the features of NPLC x RPLC, several shortcomings related with the solvent incompatibility between the two LC modes were identified and the practical consequences were discussed.

Construction of a new interface for comprehensive supercritical fluid chromatography×reversed phase liquid chromatography (SFC×RPLC)

“…The highest interaction was observed for C 18 . This was tested offline, by injecting 5,7-dimethoxycoumarin in isocratic SFC (5% EtOH), mixing the effluent with water at a flow rate of 1. valve manually at different times after SFC elution.…”

Theory and Practice of UHPLC and UHPLC–MS

“…This has led the 2D separations community to describe such combinations of separation modes as "incompatible" (Kivilompolo et al, 2011). In fact, 2D-LC separations involving such combinations of modes have been demonstrated François et al, 2006), but at great cost in terms of detection sensitivity. In these cases, the injected volume of 1 D effluent must be a small fraction of the 2 D column volume to avoid serious deterioration of the 2 D column performance.…”

Column selectivity for two‐dimensional liquid chromatography

“…Here, high proportion of hexane in the fractions transferred from the first (NP) dimension and gradients starting in pure water in the second (RP) dimension efficiently suppressed band broadening and distortion caused by mobile phase incompatibility in the two dimensions [156]. When this 2-D setup was applied for the separation of a drug mixture (containing neutral, acid and basic compounds) using isocratic elution with a mobile phase comprised of hexane, 1-butanol, and 0.2% ethanolamine, system peak and a bump originating in poor mixing of solvents were observed, which decreased the available space for separation in the second dimension [156].…”

“…When this 2-D setup was applied for the separation of a drug mixture (containing neutral, acid and basic compounds) using isocratic elution with a mobile phase comprised of hexane, 1-butanol, and 0.2% ethanolamine, system peak and a bump originating in poor mixing of solvents were observed, which decreased the available space for separation in the second dimension [156].…”