Determination of glutamate and aspartate in microdialysis samples by reversed-phase column liquid chromatography with...

Five different systems for fast determination of aspartate and glutamate in microdialysis samples are described: (I) a high-speed HPLC using a gradient pump with a sharp elution profile, (II) a column switching technique, (III) an isocratic pump with a low-pressure switching valve for one-step gradients, (IV) microbore chromatography using injections of acetonitrile as a wash-out step, (V) on-line connection of microdialysis and HPLC/derivatization. In all cases, automated precolumn derivatization with o-phthalaldehyde-2-mercaptoethanol reagent were used. Both fluorescence and electrochemical detection techniques were evaluated in terms of reproducibility, sensitivity, interference, maintenance and troubleshooting. The electrochemical detection method required a second derivatization step with 0.2 M iodoacetamide to remove excess of a thiol moiety and regular recalibrations after each six to ten injections. Under these conditions the correlation coefficients for electrochemical vs. fluorescence detectors were 0.918 for Asp and 0.988 for Glu for 65 microdialysis samples. Coefficients of variation for six analyses between calibrations were below 3% for both detectors. The limits of detection for both amino acids were about 0.4 pmol for electrochemical detection with a thiol scavenger step, 50 fmol for fluorescence detection using conventional columns and about 20-30 fmol for the microbore system. All systems are suitable for detecting basal levels of Asp and Glu in 5-10 microl microdialysis samples from a rat brain where typical concentrations lie around 1-10 pmol or more. It is concluded that a microbore setup with one isocratic pump and an autosampler optimized for injections of washing solvent between samples is the most practical and economical. The system allows analysis of minute sample volumes down to 1-2 microl.