Ethylammonium formate as an organic solvent replacement for ion-pair reversed-phase liquid chromatography

Ethylammonium formate (EAF), an inexpensive and easily synthesized room-temperature ionic liquid, acts like a conventional organic solvent for reversed-phase liquid chromatography (LC). In this report, the use of standard ion-pair reagents with this ionic liquid LC mobile phase and a polystyrene-divinylbenzene PRP-1 column is explored. Starting with the column equilibrated with a methanol mobile phase, the required equilibration time of the column by the EAF ion-pair mobile phase is determined by the plate number profile. Chromatograms of six aromatic carboxylic acids, with either methanol or EAF as the mobile phase, at room temperature (in the absence of an ion-pairing agent) lack resolution with significant peak overlap of nitro-substituted benzoic acids. The addition of 30mM tetrabutylammonium ion to the EAF or methanol mobile phase provides baseline resolution for all peaks in approximately 10 min. Analogous studies using a mixture of four aromatic amines, including protonated tyramine, diphenhydramine, and neutral nitroanilines in the absence or presence of 30mM sodium dodecylsulfate (SDS) in the mobile phase are similar to those for the aromatic acids, indicating baseline resolution with only the ion-pair reagent. Raising the column temperature to 55 degrees C improves the plate count by a factor of approximately 1.2 when using the EAF mobile phase. The retention factor profiles for either the carboxylic acids or the amines, as a function of the organic modifier percentage or ion-pair reagent concentration, are similar for both EAF and methanol. The polymerized acyl monoglycinate surfactant, poly(sodium-N-undecenoyl glycinate), is used for the first time as an LC ion-interaction reagent and is about as effective as SDS for the resolution of organic amines.