|Chiral Stationary Phase Optimized Selectivity Supercritical Fluid Chromatography (SOS-SFC): a novel approach for optimizing the separation of enantiomers
|Exploiting Separation Science
|Mr Ravindra Hegade
|Prof Frederic Lynen
Prof Roman Szucs
Abstract Information :
Stationary phase Optimized Selectivity Liquid Chromatography (SOS-LC) has been successfully further developed and increasingly used in the last decade as a novel tool for the separation of solutes in a predictable way on combined stationary phases.1 Progress has been made in the extension of the approach to allow for gradient analysis2,3 and the model also proves applicable on the compressible phases used in supercritical fluid chromatography (SFC).4 Thus far the potential of the approach to facilitate the separation and purification of stereoisomers via supercritical fluid chromatography (SFC) has not been investigated, although especially in the latter case SOS-SFC could offer significant benefits to speed up the purification process or to obtain improved chiral screening of complex mixtures.
In this work phase the possibilities of isocratic chiral SOS-SFC are therefore explored. The approach combines different chiral column segments with divergent stationary phases based on the predictions made through the PRISMA model5 such that the optimal separation can be obtained in this way. An optimized column segment combination, giving the highest separation selectivity for all the enantiomers in a mixture within shortest analysis time is predicted. As a first practical step, retention times are determined in isocratic conditions by analyzing the mixture of enantiomers on different chiral stationary phases using identical chromatographic conditions, which is have been selected through experience and trial procedures.
The stationary phases used for these basic measurements should depict strongly different selectivity, as is the case between e.g. the Lux 3u Amylose 2, Cellulose 1, Cellulose 2, Cellulose 3 and Cellulose 4 columns. The retention time of the analytes on any of these phases is different due to distinct mechanism of interaction. The measured retention factors on the individual segments are then used to predict the retention on all possible column linear combination of the segments. Subsequently the solutions can be ranked in order of decreasing selectivity of the critical pair in such a way the optimal column combination can be predicted to perform the analysis within a maximum allowed predetermined time. The methodology can also be used for the positioning of selected solutes in an as broad as possible separation window to allow for much increased loadability on the column and faster purification.