Evaluation of interactions and rational choice of separation systems
Rational development of separation methods requires a good knowledge of the interactions that govern separation systems, but also simple and efficient means for comparing them to quickly choose the potentially more efficient systems for a first screening of stationary phases.
Among these systems, we are particularly interested in supercritical fluid chromatography (SFC), reversed-phase liquid chromatography (RPLC), hydrophilic interaction liquid chromatography (HILIC) and chiral separations in the SFC and HPLC modes.
Understanding these systems requires the analysis of probe molecules. For C18 stationary phases used in RPLC, three carotenoid pigments are analyzed in SFC, and a two-dimensional classification map compared all these phases simultaneously.
For other systems, a characterization approach based on LSER model (linear solvation energy relationship) is implemented: log k = eE + sS + aA +bB + vV. It requires the analysis of a large set of compounds in standard conditions, then retention data serve to calculate the coefficients that are representative of interactions (e stands for charge transfer, s for dipole-dipole, a and b for hydrogen bonding with acids and bases, v for dispersion and cavity effects). With five descriptors, this model proves efficient for achiral SFC, and a unified classification of all sorts of stationary phases is provided with the phases located on a “spider” diagram, which positions the five interaction properties in a two-dimensional space.
Complementary descriptors can be added to this model, either to take account of ionizable species (D+ and D-) for HILIC, or to take account of shape and flexibility (G and F) for chiral separations. Then a satisfactory evaluation of these chromatographic systems is obtained, and their classification.
The spider classification can be applied to compare solvent properties, and thus facilitate their choice for liquid-liquid extraction, CCC, or to better use “green” solvents.
The study of the interactions between the compound of interest, the stationary phase and the mobile phase, allows the optimization of extractions methods such as CCC, SPME, MIP-SPE, ASE, and micro-waves and the one of separation systems such as HPLC, GC, CE, SFC, HPTLC. Several analytical approaches such as free or immobilized enzyme are also used for better understanding of the enzyme-substrate or enzyme-inhibitor interactions through the determination of their kinetic and affinity constants.