Comprehensive two-dimensional liquid chromatography with non-aqueous hydrophilic interaction conditions enables resolution of isomers of EO/PO surfactants

The analytical toolbox used to characterize surfactant mixtures in support of research, development, and manufacturing includes liquid and gas chromatography, spectroscopic techniques, and mass spectrometry. Two-dimensional liquid chromatography (2D-LC) is particularly well suited to the challenge of characterizing these mixtures because separations with complementary selectivities can be coupled to resolve molecules according to distributions of two or more functional groups. In this work we have discovered mobile phase conditions that can be used to resolve isomeric species present in mixtures of ethylene oxide (EO) - propylene oxide (PO) diblock copolymers. Specifically, we find that use of non-aqueous hydrophilic interaction (NA-HILIC) conditions, where the water typically used as the strong solvent is replaced by an alcohol, provide the selectivity needed to resolve copolymer isomers that only differ in the orientation of PO groups within the copolymer chain. When used in a 2D-LC method, these conditions enable highly selective separation of hundreds of peaks, both by the number of EO (first dimension) and PO (second dimension) groups, and according to PO isomer structure. Within analysis times of about four hours, we achieved peak capacities on the order of 1000, which we believe is a significant step forward in the characterization of EO-PO copolymers relative to prior published work and will be very useful for understanding the composition and behavior of these mixtures going forward.