On-line concentration and separation of cationic and anionic neurochemicals by capillary electrophoresis with UV absorption detection.

This paper presents on-line simultaneous concentration and separation of cationic and anionic neurochemicals by capillary electrophoresis (CE) with UV absorbance spectroscopy. Neurochemical stacking exploits differences in local electric field and viscosity between the sample zone and the background electrolyte (BGE). To achieve these discontinuous conditions for CE, neurochemicals were prepared in a solution containing 1mM formic acid and 20% (v/v) acetonitrile (ACN). The capillary was filled with a solution of 500 mM Tris-borate (TB) and 10% (v/v) glycerol. The buffer vial contained 500 mM TB and 0.5% (v/v) polyethylene oxide (PEO). After injecting a large sample volume, PEO enters the capillary by electro-osmotic flow (EOF). Anionic neurochemicals stacked at the sample zone and PEO-containing BGE boundary. Simultaneously, cationic neurochemicals were concentrated at the boundary between the sample zone and the glycerol-containing BGE. The concentrated cationic neurochemicals were baseline separated in the presence of glycerol, mainly due to hydrogen bonding interactions between glycerol hydroxyl groups and the neurochemical's hydroxyl and amino groups. Under optimal stacking conditions, we observed the following: (a) the maximum sample injection volume was 720 nL; (b) the limit of detection for signal-to-noise ratio of 3 ranged from 14.7 to 313.4 nM; and (c) sensitivity enhancements compared to normal injection (15 nL) ranged from 116 to 281-fold. We evaluated the proposed method by the determination of neurochemicals in urine samples.