Oxalic acid stabilizes dopamine, serotonin, and their metabolites in automated liquid chromatography with electrochemical detection.

Use of antioxidative agents is required in automated LC assay of microdialysis samples, due to rapid degradation of the monoamine neurotransmitters and their metabolites. Addition of oxalic acid prevented degradation of dopamine, serotonin, 3,4-dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindoleacetic acid efficiently: after a 24-h incubation at room temperature the decreases in peak heights were less than 10%. The long-term stability of the analytes, however, was still enhanced when acetic acid and L-cysteine were included in the solution. Using this antioxidative solution, the monoamine neurotransmitters and their metabolites could be determined with an automated LC assay even at room temperature.