n-Electrode three-dimensional liquid chromatography with electrochemical detection for determination of neurotransmitters.

Coulometric electrodes in series have been used with liquid chromatography with electrochemical detection to increase selectivity and resolution for the direct analysis of tissue neurotransmitters. Use of three coulometric sensors for electrochemical modification, selectivity, and peak identification has been expanded into "gate" cells of three or four coulometric electrodes that allow elimination of all electrochemically irreversible substances, and "array" cells of up to 15 coulometric electrodes for separation of co-eluting compounds by their current/voltage characteristics. On-column sensitivity of the sensor arrays is 0.4 to 4 pg. Gate cell selectivity favors electrochemically reversible compounds over irreversible ones, e.g., 3-methoxy-4-hydroxyphenylglycol vs ascorbate, by a factor of up to 10(4). Resolution across the multi-electrode array cells allows separation of co-eluting compounds with half-wave potentials differing by as little as 30 to 40 mV. Cells with three to 15 electrodes have been used to measure monoamines and metabolites in brain; monoamines directly in serum filtrate; and the state of oxidation of 5-hydroxytryptamine and 5-hydroxytryptophan in cerebrospinal fluid of patients with Alzheimer's disease.