Fumihiko Kitagawa

This review highlights on-line sample preconcentration by large-volume sample stacking with an electroosmotic flow pump (LVSEP) in microscale electrophoresis. LVSEP requires only a hydrodynamic sample injection into the whole capillary/microchannel and a constant voltage application, i.e., without polarity switching, providing an efficient sensitivity enhancement with a relatively easy experimental procedure without loss of the separation efficiencies. Recent developments and applications of LVSEP are briefly reviewed from the first LVSEP study in 1999 with categorization on the basis of separation modes, including zone electrophoresis, cyclodextrin- and affinity-based separation modes and non-aqueous capillary electrophoresis.

The preparation of cinchonidine (CCND) nanocrystals and their immobilization onto the inner surface of a fused silica capillary was investigated for use in the enantioseparation by capillary electrochromatography. The CCND nanocrystals were prepared by an emulsion method that utilizes crystal growth in an oil-in-water emulsion without a stabilizer. As a result, white-colored aqueous dispersions of organic nanoparticles were obtained. SEM images showed that the prepared CCND nanocrystals were from 300 to 700 nm in size. When the obtained dispersion was introduced into a poly(diallyldimethylammonium chloride) (PDDAC) coated capillary, the nanocrystals were strongly adsorbed onto the inner surface due to an electrostatic interaction between cationic PDDAC and the negatively charged organic nanocrystals. In CCND nanocrystals coated capillary, CEC enantioseparations of racemic amino acids were successfully achieved. The reproducibilities of the detection times were acceptable with a relative standard deviation of less than 7%, indicating that stable nanocrystal coating could be obtained by our proposed method.

This review highlights recent developments and applications of on-line sample preconcentration techniques to enhance the detection sensitivity in microchip electrophoresis (MCE); references are mainly from 2008 and later. Among various developed techniques, we focus on the sample preconcentration based on the changes in the migration velocity of analytes in two or three discontinuous solutions system, since they can provide the sensitivity enhancement with relatively easy experimental procedures and short analysis times. The characteristic features of the on-line sample preconcentration applied to microchip electrophoresis (MCE) are presented, categorized on the basis of “field strength-” or “chemically” induced changes in the migration velocity. The preconcentration techniques utilizing field strength-induced changes in the velocity include field-amplified sample stacking, isotachophoresis and transient-isotachophoresis, whereas those based on chemically induced changes in the velocity are sweeping, transient-trapping and dynamic pH junction.