Dual-opposite injection electrokinetic chromatography for the unbiased, simultaneous separation of cationic and anionic compounds

The concept of dual opposite injection in capillary electrophoresis (DOI-CE ) for the simultaneous separation, under conditions of suppressed electroos motic flow, of anionic and cationic compounds with no bias in resolution an d analysis time, is extended to a higher pH range in a zone electrophoresis mode (DOI-CZE). A new DOI-CE separation mode based on electrokinetic chrom atography is also introduced (DOI-EKC). Whereas conventional CZE and DOI-CZ E are limited to the separation of charged compounds with different electro phoretic mobilities, DOI-EKC is shown to be capable of separating compounds with the same or similar electrophoretic mobilities. In contrast to conven tional EKC with charged pseudostationary phases that often interact too str ongly with analytes of opposite charge, the neutral pseudostationary phases appropriate for DOI-EKC are simultaneously compatible with anionic and cat ionic compounds. This work describes two buffer additives that dynamically suppress electroosmotic flow (EOF) at a higher pH (6.5) than in a previous study (4.4), thus allowing DOI-CZE of several pharmaceutical bases and weak ly acidic positional isomers. Several DOI-EKC systems based on nonionic (10 lauryl ether, Brij 35) or zwitterionic (SB-12, CAS U) micelles, or nonioni c vesicles (Brij 30) are examined using a six-component test mixture that i s difficult to separate by CZE or DOI-CZE. The effect of electromigration d ispersion on peak shape and efficiency, and the effect of surfactant concen tration on retention, selectivity, and efficiency are described.