Published:Journal of Chromatographic Science,
ISSN 0021-9665 Volume 44,
Number 10, November/December 2006, pp.607-614
Ethylammonium Formate as an Organic Solvent Replacement
for Ion-Pair Reversed-Phase Liquid Chromatography
Martin M. Waichigo and Neil D. Danielson
Department of Chemistry and Biochemistry, Miami University, Oxford,
OH 45056
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°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.
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