Determination of gamma-aminobutyric acid in microdialysis samples by microbore column liquid chromatography and fluorescence detection.

A method for the determination of gamma-aminobutyric acid (GABA) at concentrations as low as 1.5 nM in microdialysis samples is described. A high-performance liquid chromatography (HPLC) autosampler was used for precolumn derivatizations of samples with phthalaldehyde-2-mercaptoethanol reagent, as well as for automated injections of acetonitrile to wash the microbore column between injections. This permitted the use of only one isocratic pump and reduced the run-to-run analysis time to below 20 min, without any risk for late-eluting peaks. Three different microbore columns were evaluated. The optimal column was a 150 x 1 mm I.D., C18 reversed-phase silica column with a particle size of 3 microm, operated at a flow-rate of 50 microl/min. The mobile phase was 0.1 M sodium acetate buffer, pH 5.4, containing 20% acetonitrile. Under these conditions, the column exhibits a low back-pressure of about 11.1 MPa and is usable for at least 1000 injections. Good correlation was found between fluorescence and electrochemical detector responses for o-phthaldialdehyde-GABA derivatives, although the latter detector type generally required more skill and maintenance for routine use. Microdialysis samples of 10-20 microl, recovered from various rat brain nuclei, were assayed using the described method.