Green approaches to comprehensive two-dimensional liquid chromatography (LC x LC)

Comprehensive two-dimensional liquid chromatography (LCxLC) x LC) has been adopted in the analysis of complex mixtures because it can achieve much higher peak capacity and resolution than one-dimensional liquid chromatography. The ever-growing demand for protecting the environment necessitates reducing or eliminating hazardous solvents to improve the environmental friendliness of analytical procedures. Acetonitrile is one of class II solvents which are hazardous to the environment and should be phased out and replaced by eco-friendlier alternatives. Herein, we introduce the use of propylene carbonate as an eco-friendly mobile phase component in comprehensive two-dimensional liquid chromatography to separate a mixture of 39 pharmaceutical compounds. Two sets of experiments were developed. In the first set of experiments, two emulated on-line LC x LC systems were developed using Solvent X (propylene carbonate: ethanol, 60:40) as a green organic modifier in the first dimension (1 1 D) and ethanol in the second dimension (2 2 D). Upon comparing the separation performance of these two systems to two systems described in the literature that used acetonitrile and methanol instead, no significant differences in terms of selectivity, peak capacity and orthogonality were found. In the second set of experiments, three comprehensive LC x LC systems were developed using Solvent X or acetonitrile in the first dimension, while ethanol or methanol were used in the second dimension. The same stationary phases were used in the three systems, but the organic modifiers were changed in one or both dimensions in order to confirm that acetonitrile can be replaced by a green alternative without losing separation efficiency. There was no significant difference in terms of selectivity, peak capacity and orthogonality between the three systems.