Kinetic optimisation of the reversed phase liquid chromatographic separation of proanthocyanidins on sub-2 μm and superficially porous phases

Phenolic compounds, and proanthocyanidins in particular, are important natural molecules which are of significant importance due to their sensory and biological activities. The analysis of proanthocyanidins in natural products is very challenging due to their complex nature. In this study, the kinetic performance of a range of recently developed C18 columns, including sub-2 μm fully porous and 2.6 μm superficially porous particle-packed columns, was evaluated for improved proanthocyanidin analysis. The kinetic plot method was employed to compare the ultimate performance limits of each column in terms of efficiency and speed for different maximum pressures and temperatures using representative proanthocyanidins comprising a range of molecular weights and functionalities as test analytes. By combining plate height data with relevant parameters such as column permeability and mobile phase viscosity, plots of practically attainable efficiencies as a function of analysis time for specific experimental configurations were obtained and performance limits for all investigated supports could accurately be compared. Both fully- and superficially porous particles provided significant speed and/or efficiency gains compared to conventional 5 μm particle packed columns. Analyte properties, particle size and packing quality as well as analysis temperature were all found to have a significant influence on the performance of the presently investigated chromatographic supports. For smaller compounds, higher optimal linear velocities and better performance in the low-efficiency range were observed, while the lower diffusion coefficients of larger procyanidins resulted in lower optimal linear velocities and better performance in the high-efficiency regime. Faster analyses become possible at higher temperatures due to decreased eluent viscosity and faster mass transfer, which was especially beneficial for larger compounds and resulted in dramatic improvement in efficiency. A possible explanation of the abnormal behaviour of oligomeric procyanidins is presented. Our findings indicate that new column formats, when used under optimal conditions, significantly improve the speed and/or efficiency of reversed phase liquid chromatographic analyses of proanthocyanidins.