Site-specific sampling of taurine from rat brain followed by on-line sample pre-concentration, throughout in-capillary derivatization and capillary electrophoresis

A method of pinpoint-sampling followed by on-line pre-concentration of the sample, throughout in-capillary derivatization and capillary electrophoretic separation was evaluated by demonstrating the detection of taurine, 2-aminoethanesulfonic acid at a specific location of a rat brain. The direct sampling of taurine from the rat brain was accomplished by using voltage injection associated with two kinds of driving forces, electrophoretic flow and electroosmotic flow (EOF). The capillary tube (75 μm of inner diameter × 375 μm of outer diameter) of the capillary electrophoresis (CE) apparatus was already filled with a CE run buffer, viz., 40 mM phosphate–borate buffer (pH 10) containing 2 mM o-phthalaldehyde (OPA)/N-acetylcysteine (NAC) as the derivatization reagent. One end of a platinum wire (0.5 mm o.d.), used as the anode, and the inlet end of capillary tube (from which a 1.0 cm long polyimide coating was removed), were pricked down onto the surface of either the cerebrum or cerebellum of a rat brain at a location of very small dimension. When a low voltage (5 kV, 30 s) was applied, taurine began to move from the rat brain into the capillary tube, and, simultaneously, electric focusing of taurine occurred by the action of “the pH-junction effect” at the inlet end of the capillary tube. After completing the injection, both the platinum wire and capillary tube were detached from the brain and dipped into the run buffer in an anode reservoir filed with the same solution as that in the capillary tube for the CE apparatus. Then, by applying a high voltage (20 kV) between the ends of the capillary tube, taurine was automatically derivatized to yield the fluorescent derivative, separated and detected with fluorescence (Ex = 340 nm, Em = 455 nm) during migration throughout the capillary tube. The migration profiles obtained from cerebrum and cerebellum appeared to be different, but the peak corresponding to taurine was identified on both electropherograms. The efficacy of the present method including sample on-line pre-concentration prior to throughout in-capillary derivatization CE was first verified with several preliminary experiments by using samples of taurine in water, saline and a piece of 1.5% agar–gel block, as an alternate standard for the rat brain used in this study.