Separation of imidazolium-based ionic liquid cations with various substituents on a pentafluorophenylpropyl stationary phase using UHPLC-MS.

Abstract

Ionic liquid cations are inherently fixed-charge and, when paired with smaller or polar substituents, not well-suited for reverse phase columns; they are also bulky organic species which can make methods traditionally used for monoatomic cation analysis less than ideal. For aromatic IL cations, such as those built upon the popular imidazolium scaffold, the pentafluorophenylpropyl (PFPP) column allows π-π stacking to improve retention. The PFPP stationary phase also has the potential to better retain polar substituents via additional dipole-dipole and/or H-bonding interactions. Previous related studies have shown good retention of alkyl and benzyl imidazolium cations on the PFPP stationary phase, using UV/Vis as a detector. The aim here is to (1) evaluate the PFPP stationary phase for methyl-imidazolium cations with a more chemically diverse set of substituents, specifically including polar groups, that mimic task-specific IL design, and (2) prioritize compatibility of mobile phases with electrospray-mass spectrometric detection to facilitate improved selectivity and sensitivity. Elution orders for six IL cations were determined across several mobile phase compositions. It was found that all six IL cations were adequately retained on the PFPP column; that acetonitrile performed better than methanol as a mobile phase organic modifier; and that 20/80 acetonitrile/water with 0.1% formic acid allowed for acceptable separation of all six analytes. Further, it was established that, under these experimental conditions, the nature of the anion contributed negligibly to the observed retention times. This provides promising results for the broad suitability of PFPP-based separations of imidazolium IL cations across the scope of synthetic tunability.