Application of nonaqueous capillary electrophoresis to the simultaneous analysis of anionic surfactants

Abstract

In aqueous capillary electrophoresis selectivity between different alkyl chain lengths within one anionic surfactant-group markedly exceeds selectivity between different functionalities at a given chain length. Peak identification and quantitative analysis in complex mixtures is almost impossible, especially, if the sample contains ethoxylated surfactants as well. Applying nonaqueous capillary electrophoresis (NACE), significant differences in the mobilities of the various functionalities can be generated to exceed at satisfying separation. In this paper, method development of NACE systems is described and the application of these systems to anionic surfactant analysis in real sample matrices is documented.

Characteristics of the electroosmotic flow of electrolyte systems for nonaqueous capillary electrophoresis

“…Electroosmotic mobilities (m eo ) were calculated from m eo = Ll/(t eo V), where L is the total capillary length in cm, l is the injection-to-detection length in cm, t eo is the migration time for the EOF-marker in s, and V is the separation voltage in V. Anthracene was chosen as the EOF-marker, because its three distinct local absorption-maxima (340, 360, and 380 nm) were easily identified in all organic solvents using direct UVdetection. A 10 mM stock solution of anthracene was prepared in 100% ACN and diluted ten-fold in the different organic solvents with 5 mM NH 4 OAc. The addition of NH 4 OAc to the organic solvent increased the conductivity of the injected sample plug and promoted a stable current.…”

“…A 10 mM stock solution of anthracene was prepared in 100% ACN and diluted ten-fold in the different organic solvents with 5 mM NH 4 OAc. The addition of NH 4 OAc to the organic solvent increased the conductivity of the injected sample plug and promoted a stable current. Before EOF measurements were performed, the capillary was rinsed with 1 M NaOH for 2 min, followed by deionized water for 2 min, 0.1 M HCl for 2 min, deionized water for 2 min, MeOH for 2 min and the BGE for 4 min.…”

“…The choice of the electrolytes was related to studies of suitable organic separation systems for the analysis of surface-active substances [4].…”

“…The organic solvent systems in these studies were selected due to their compatibility for the analysis of surface-active substances [4]. NH 4 OAc and four background chromophores (BGCs) were studied in MeOH, ACN, FA, NMF, DMF, and DMSO.…”

“…NH 4 OAc was investigated, because it is a suitable electrolyte for direct UV-detection of anions and cations in the six organic solvents. Additionally, it does not cause proteolysis, as OAc 2 is a weak conjugated base and NH 4 1 is a weak conjugated acid. For indirect UV-detection, the best peak shapes are obtained when the BGC and the analyte have comparable mobilities.…”