Journal of Chromatography A, 652 (1993) 399-405 Elsevier Science Publishers B.V.. Amsterdam CHROM. 24 974 Collinear laser-induced fluorescence detector for capillary electrophoresis Analysis of glutamic acid in brain dialysates* Luis Hernandez* Los Andes University, Apartado 109, Merida 5101A (Venezuela) and Europhor Instruments, Part Technologique du Canal, 10 Avenue de I’Europe, 31520 Ramonville (France) Narahari Joshi and Euro Murzi Los Andes University, Apartado 109, Merida 5101A (Venezuela) Philippe Verdeguer and Jean Christophe Mifsud Europhor Instruments, Part Technologique du Canal, 10 Avenue de I’Europe, 31520 Ramonville (France) Norberto Guzman Princeton Biochemicals Ind., Princeton, NJ (USA) ABSTRACT Experiments with capillary electrophoresis using a laser-induced fluorescence detector with a collinear optical arrangement demonstrated several important points. First, increasing the numerical aperture of the microscope objective that is used simultaneously for focusing the excitation laser light as well as collection of emitted fluorescence enhances the signal used for the measurement of the emitted fluorescence and at the same time decreases the noise of interfering light. Second, detection of fluorescein-labelled amphetamine was performed at high-picomolar (lo-” M) levels. Third, the signal-to-noise ratio of 280 found at the above-mentioned picomolar concentrations indicates that the measurement of low-picomolar concentrations (lo-‘* M) of this compound in biological samples should be possible. Fourth, narrow-bore capillaries (5-10 ym internal diameter) were used to detect the neurotransmitters glutamic acid and aspartic acid as their naphthalene-2,3-dicarboxaldehyde derivatives in brain dialysates obtained from a freely moving rat. A mathematical model was developed to explain the relationship between numerical aperture, working distance, magnification of the lens, noise due to laser scattering and signal due to fluorescence. The model correctly predicted the observed values of photomultiplier tube current due to both laser scattering and fluorescence. The potential of the application of capillary electrophoresis with laser-induced fluorescence detection in the neurosciences is discussed. * Corresponding author. Address for correspondence: Los Andes University, Apartado 109, Merida 5101A, Venezuela. * Presented at the 4th International Sy mposium on High Performance Capillary Electrophoresis, Amsterdam, February 9- 13, 1992. The majority of the papers presented at this symposium were published in J. Chromatogr., Vol. 608 (1992). 0021-9673/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved