Characterization and quantitation of mixtures of alkyl ether sulfates and carboxylic acids by capillary electrophoresis with indirect photometric detection

Abstract

The separation, characterization, and determination of mixtures of alkyl ether sulfates (AES) and fatty acids (C10-C16) in background electrolytes (BGEs) containing acetonitrile (ACN)-water mixtures is addressed. Due to inhibition of the ionization of the carboxylate groups, the migration time and the resolution between the fatty acids decreased when the water content of the BGE was reduced, but efficiency and resolution between the AES oligomers improved. The migration times increased and resolution improved by substituting 5% ACN by an equivalent amount of dioxane. A complete separation of the two surfactant classes, up to the AES oligomers with 8 ethylene oxide units (EOs) with respect to C10, with excellent resolution between the AES oligomers, while preserving a satisfactory resolution between the fatty acids, was achieved with a BGE containing 5 mM trimethoxybenzoic acid, 7 mM dipentylamine, 85% ACN, 5% dioxane, and 10% water. The two surfactant classes were increasingly resolved by further reducing the water content of the BGE. Thus, C2 (acetate) was resolved from the AES oligomers up to 7 EOs using 90% ACN and 5% dioxane, but the resolution between the heavier fatty acids was poor with this BGE. Identification of the AES oligomers was eased by the excellent regularity of the successive migration times; thus, within each AES subclass or series of oligomers with the same number of carbon atoms in the alkyl chain, the migration times decreased following a mild curve as the number of EOs increased. The way how the data obtained by indirect photometry (corrected peak areas that are proportional to the molar concentrations) should be managed to avoid systematic error when the calibration curve is constructed using an AES standard with an oligomer distribution different from that of the samples is discussed and equations are given. Decyl sulfate was successfully used as internal standard. The detection limits (S/N = 3) were of ca. 2 microM for individual AES oligomers.

Separation of perfluorocarboxylic acids using capillary electrophoresis with UV detection

“…This has been applied in CE determinations of various carboxylic acids in phosphate BGE to which 15 [24] to 30% [21] acetonitrile was added, for the determination of dicarboxylic acids with 40% ethanol added [21], and for fatty acids with ammonium acetate buffer and 50% acetonitrile added [28]. Determinations of anionic surfactants required 50% methanol in BGE [23], whereas determinations of fatty acids with BGEs containing trimethoxybenzoic acid and dipentylamine with addition of 85% acetonitrile and 5% dioxane were reported [30].…”

“…A convenient on-line method of improvement of detection limits, developed for capillary electrophoresis of charged analytes, is stacking that occurs when ions cross boundary separating regions of a high electric field sample zone and a low electric field BGE [30][31][32][33][34][35][36][37][38][39][40]. The volume of sample injected prior to the stacking phase affects significantly broadening of the signal.…”

“…The indirect UV detection requires the addition of an appropriate chromophore agent to the BGE. Various chromophores have been used for indirect UV detection of carboxylic acids: naphthalene sulfonates [22,23], 2,6-naphthalenedicarboxylic acid [31], trimethoxybenzoic acid [32], 2,6-pyridinedicarboxylic acid [33,34,35], 3,5-dinitrobenzoic acid [36], 5-sulfocalicylate [37], and chromate [38], commonly used for indirect UV detection of anions in CE.…”

Coceth sulfate characterization by electrospray ionization tandem mass spectrometry

“…Alkyl ether sulfates (AESs), belonging to this group of tensioactive agents, can be used in hard waters and are capable of maintaining foaming at low temperatures. These features make them suitable for use in most cleaning formulations 1,2,8 …”

“…From the chemical structural point of view, AESs are formed of a linear alkyl chain of carbon atoms, which represents the hydrophobic tail, with a hydrophilic head composed of a variable number n of ethylene oxide groups (EOs) with a terminal sulfate group 8,9 . AESs are prepared by reacting ethoxylated alcohols with a sulfating agent, normally sulfur trioxide; feedstocks for ethoxylated alcohols are made from synthetic or natural fatty acids, with carbon chain lengths between C6 and C18 depending on the required applications 9 .…”