Dual opposite injection electrokinetic chromatography: Nonionic microemulsion pseudostationary phase and novel approach to electrokinetic sampling bias

Abstract

Dual opposite injection capillary electrophoresis (DOI-CE) is a family of CE techniques in which the sample is introduced into both ends of the capillary. For the analysis of compounds with widely varying pKa values using a voltage-driven separation scheme, DOI-CE is superior to conventional CE with sample introduction at only one end of the capillary due to DOI-CE's broader elution window. To enhance the DOI-CE separation, a running buffer with a microemulsion system was developed. Since DOI-CE works best under conditions of low electroosmotic flow (EOF), the suppression of EOF via the addition of a multiply charged cation (e.g., Zn2+) to the buffer was investigated, and was found to suppress the EOF effectively at moderate concentrations (2.5-10 mM). Three different dual opposite injection modes were studied: simultaneous electrokinetic injection, sequential electrokinetic injection, and sequential hydrodynamic injection. The injection bias in the first two electrokinetic injection modes was compared with the sequential hydrodynamic injection. Corrections in the bias of the electrokinetic injections were discussed, and an improved approach was suggested. Finally, the effect of the relative concentration of the multiply charged cation in the sample plug and running buffer on the peak shape of co-electroosmotic and counter-electroosmotic ions was examined, and found to be much more influential on the latter.

Recent advances in microemulsion electrokinetic chromatography

“…They have been used to separate insoluble sun-tan lotion additives [39], a range of methylquinolines [20], and to provide optimum conditions for a MEEKC-dual opposite injection separation [40]. Mixed surfactant systems, where two different types of surfactant are used to provide the optimum separation selectivity, have been reported [6,26,41].…”

“…Dual opposite injection is a sampling technique [40] where cations and anions are injected and migrate from opposite ends of the capillary toward the detector. The detection window can be situated in the middle of the capillary, which eliminates the analysis time between cationic and anionic compounds, offering the possibility of greater resolution without increasing run times.…”

Recent advances in the development and application of microemulsion EKC

“…The detection window can be situated at the middle of the capillary, thus compensating for the different migration times of cationic and anionic compounds, offering the possibility of greater resolution without increasing run times. Zhou and Foley [38], investigated DOI-MEEKC using a low EOF facilitated by incorporating zinc cations in the microemulsion electrolyte and a neutral surfactant to achieve partitioning of the solutes.…”

“…For the analysis of compounds with widely varying pK a values using a voltage-driven separation scheme, DOI-CE is superior to conventional CE (with sample introduction at only one end of the capillary) due to a broader elution window [38]. The detection window can be situated at the middle of the capillary, thus compensating for the different migration times of cationic and anionic compounds, offering the possibility of greater resolution without increasing run times.…”

“…They can however be added to microemulsions without increasing the overall operating current. They have been used to separate insoluble sun-tan lotions and to provide optimum conditions for MEEKC dual-opposite injection separation [38].…”