One- and comprehensive two-dimensional high-performance liquid chromatography analysis of alkylphenol polyethoxylates

High-performance liquid chromatography (HPLC) analysis of alkylphenol polyethoxylates (APnEOs) in one-dimensional (1-D) mode usually separates either the ethoxy or alkyl moiety distribution and requires different modes for adequate resolution. Simultaneous complete separation of variable ethoxymer chain lengths and variable alkyl end groups using 1-D-HPLC has not been reported and suggests that multidimensional (MDLC) or comprehensive two-dimensional liquid chromatography (LC×LC) might offer a suitable separation approach for this goal. This study compares different separation modes--normal phase LC (NPLC), reversed-phase LC (RPLC) and hydrophilic interaction chromatography (HILIC)--in terms of separation for alkyl and ethoxy distributions. RPLC provided adequate separation of octyl and nonyl APnEOs using an isocratic elution program and was selected as second dimension ((2)D) for LC×LC. NPLC offered better resolution than HILIC; however, non-polar NPLC solvent immiscibility with RPLC mobile phases leads to HILIC being chosen as first dimension ((1)D). The HILIC×RPLC system was evaluated by analysis of four APnEO mixtures. Complete simultaneous separation of APnEOs into individual oligomers, with each alkyl end group resolved, demonstrated the capability of the LC×LC method. Different descriptors and metrics for assessing system orthogonality were investigated to evaluate HILIC×RPLC performance. A relatively high dimensionality of 1.76 was demonstrated.