Unexpected Solvent Selectivity Effects Encountered in Two-Dimensional Liquid Chromatography Separations of Non-Ionic Copolymer Surfactants

Analysis of non-ionic copolymer surfactants is challenging due to complexities of the mixture of molecules that results from industrial-scale synthesis involving heterogeneous starting materials. Two-dimensional liquid chromatography (2D-LC) involving hydrophilic interaction (HILIC) and reversed-phase (RP) separation modes has been shown to be particularly effective for this purpose. The HILIC mode is highly selective for the separation of oligomers that vary in the number of hydrophilic (e.g., ethylene oxide [EO]) repeating units, and the RP mode is highly selective for the number of hydrophophic repeating units, such as polypropylene oxide (PO). In our work on separation of EO/PO copolymer mixtures by 2D-LC we have discovered that certain mobile phase compositions used for the HILIC separation yield unexpected selectivities for isomers of these copolymers that can result from differences in the way the EO/PO oligomers are assembled during the synthesis reaction. In this presentation we will share results of a systematic study of mobile phase constituents that exhibit these selectivity effects, and demonstrate the utility of these new-found effects in ultra-high resolution 2D-LC separations of EO/PO copolymers showing that they provide insights into the species-level composition of these complex mixtures that was not possible previously.