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 approximately 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 microL/min by placing a 360-microm-diameter fluidic access hole that had flow resistance (0.15-7.2) x 10(8)-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-trans-pyrrolidine-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.

Mesoporous silica that was prepared by a sol-gel process was used as the electrolyte for the amperometric determination of vapor-phase monomethyl hydrazine (MMH). Three detection strategies were tested, namely cyclic voltammetry at a Pt working electrode with the oxidation mediated by Fe II,III that was hosted by the silica, pulsed electrochemical detection with a gold working electrode, and potentiostatic oxidation at a mixed-valence ruthenium oxide (mvRuOx) ®lm that was deposited on a Pt working electrode prior to casting the solid electrolyte overlayer. The sensitivities (and linear dynamic ranges) for the respective methods were 1.4 nA/ppm (500±1400 ppm), 0.49 nA/ppm (70±1050 ppm), and 0.30 nA/ppm (1±1050 ppm). With pulsed electrochemical detection and mvRuOx, respective detection limits of 20 ppm and 0.3 ppm were calculated on the basis of a concentration that yielded a response three times the standard deviation of the blank. Because of high uncertainty of the intercept of the Fe II,III -mediated method, a detection limit was not calculated for this technique. A cell design that provided a three-phase boundary at a mvRuOx-modi®ed electrode yielded a response of 90 % of the steady-state current at 11 s.

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A sensor system for the automatic, in-line, determination of chlorite ion is reported. Electroanalytical measurements were performed in electrolyte-free liquids by using an electrochemical probe (EC), which enables in-line detection in high-resistance media such as disinfected water. Cyclic voltammetry scan rate studies suggest that the current arising from the oxidation of chlorite ion at an EC probe is mass-transfer limited. By coupling FIA with an EC probe amperometric cell, automated analysis was achieved. This sensor is intended to fulfill the daily monitoring requirements of the EPA DBP regulations for chlorite ion. Detection limits of 0.02-0.13 mg/L were attained, which is about one order of magnitude below the MRDL. The sensor showed no faradaic signal for perchlorate, chlorate, or nitrate. The lifetime and stability of the sensor were investigated by measuring calibration curves over time under constant-flow conditions. Detection limits of <0.1 mg/L were repeatedly achieved over a period of three weeks.