Characterization of carbon paste electrodes in vitro for simultaneous amperometric measurement of changes in oxygen and ascorbic acid concentrations in vivo.

Differential-pulse amperometry, an established technique for the in vivo monitoring of dopamine in brain extracellular fluid (ECF), was extended to the simultaneous electrochemical detection of molecular oxygen (O2) and ascorbic acid (AA). Lipid-treated carbon paste electrodes (LCPEs) were characterized in vitro using this technique and found to be ideally suited for the detection of both compounds. For O2 reduction, two equally sized cathodic pulses were applied, the first from a resting potential at -150 to -350 mV, which corresponds to the foot of the reduction wave for O2 at LCPEs, and the second from -350 to -550 mV, which corresponds to the peak of the reduction wave. Following the same criterion, equally sized anodic pulses were then applied from -150 to +50 mV and from +50 to +250 mV for AA oxidation. The complete sequence of pulses for O2 and AA detection lasts 1 s. Interference by O2 with AA currents and vice versa was not a problem. Also, several compounds present in brain ECF were tested and shown not to interfere appreciably with the amperometric signal for either compound. The technique was tested in vivo, and results from behavioural stimulation, achieved by the application of tail pinch, support the conclusion of simultaneous independent detection of changes in O2 and AA at LCPEs.