Determination of quaternary ammonium compounds by capillary electrophoresis using direct and indirect UV detection

Abstract

The determination of alkyl and alkylbenzyl quaternary ammonium compounds can be difficult owing to the polarity of the compounds, the formation of micelles by longer chain compounds and the lack of a chromophoric substituent for detection by standard separation techniques. The development of a free zone capillary electrophoresis method necessitated the use of organic modifiers in order to disrupt the formation of micelles by the longer chain (C 12) surfactants. The selectivity obtained by using various levels of tetrahydrofuran as an organic modifier allowed the separation of a mixture of alkylbenzyl and alkylethylbenzyl quaternary compounds. The separation of alkyltrimethyl and dialkyldimethylammonium compounds was accomplished by the addition of a chromophoric cationic compound to the buffer system in order to allow the detection of the compounds with a standard UV absorbance detector. By varying the electrophoretic mobility of the indirect detection reagent, a variety of alkyl quaternary compounds, ranging from tetra- methylammonium chloride to stearyltrimethylammonium chloride, can be detected with good peak shapes. The use of these methods for the determination of quaternary ammonium compounds will also be discussed. The use of internal standard methods and the linearity and linear range of the technique are considered.