Microfluidic Electrophoresis Chip Coupled to Microdialysis for in Vivo Monitoring of Amino Acid Neurotransmitters

Microfluidic electrophoresis devices were coupled on-line to microdialysis for in vivo monitoring of primary amine neurotransmitters in rat brain. The devices contained a sample introduction channel for dialysate, a precolumn reactor for derivatization with o-phthaldialdehyde, a flow-gated injector, and a separation channel. Detection was performed using confocal laser-induced fluorescence. In vitro testing revealed that the initial device design had detection limits for amino acids of ~200 nM, relative standard deviation of peak heights of 2%, and separations within 95 s with up to 30 200 theoretical plates when applying an electric field of 370 V/cm. A second device design that allowed electric fields of 1320 V/cm to be applied while preserving the reaction time allowed separations within 20 s with up to 156 000 theoretical plates. Flow splitting into the electrokinetic network from hydrodynamic flow in the sample introduction channel was made negligible for sampling flow rates from 0.3 to 1.2 L/min by placing a 360-m-diameter fluidic access hole that had flow resistance (0.15-7.2) * 108-fold lower than that of the electrokinetic network at the junction of the sample introduction channel and the electrokinetic network. Using serial injections, the device allowed the dialysate stream to be analyzed at 130-s intervals. In vivo monitoring was demonstrated by using the microdialysis/microfluidic device to record glutamate concentrations in the striatum of an anesthetized rat during infusion of the glutamate uptake inhibitor L-transpyrrolidine-2,4-dicarboxylic acid. These results prove the feasibility of using a microfabricated fluidic system coupled to sampling probes for chemical monitoring of complex media such as mammalian brain.