Towards greener enantioselective liquid chromatography: An exploratory study on chiral liquid crystals

The effort for implementing more environmentally-conscious practices has also gained momentum in separation techniques. One possible approach is replacing conventional solvents with alternative solvents that are less hazardous to health/environment. Due to properties of chiral liquid crystals, their enantioselective chromatography is mostly conducted under normal mode or polar organic mode conditions with acetonitrile-rich mobile phase. It is apparent there is a room for employing less polluting solvents.

Dimethyl carbonate, propylene carbonate and dimethoxymethane (methylal) were used as mobile phase components for direct enantioseparation of six chiral liquid crystals on chlorinated polysaccharide-based chiral stationary phase (CSP). The chromatographic behaviour of the selected solutes has been previously studied in acetonitrile-based mobile phases on the same CSP which enabled a comparison of the results obtained with conventional solvents and those obtained with the greener solvents. Each of the green solvents was used instead of acetonitrile in binary mobile phases with methanol or 2-propanol. Fast baseline enantioseparations (retention factor of the late eluting enantiomer k < 2) were achieved for five out of six analytes; for the last analyte baseline enantioseparation has been achieved at longer time. Aqueous mobile phases were also tested and enabled baseline separation of three solutes. Effect of flow-rate on the separation efficiency is briefly discussed as well as suitability of the three solvents for use as sample solvents.

The is the first use of propylene carbonate and methylal in enantioselective chromatography. The green solvents were found to be suitable alternatives to acetonitrile for chiral separation of liquid crystalline materials as all six analytes were separated to baseline using acetonitrile-free mobile phases. Practical aspects of working with the selected green solvents are briefly summarized.