Analysis of tryptophan and tyrosine in cerebrospinal fluid by capillary electrophoresis and “ball lens” UV-pulsed laser-induced fluorescence detection

For the purpose of this study, we used a “ball lens” UV laser-induced fluorescence (LIF) detector comprising a pulsed laser and a collinear optical arrangement. The fluorescence signal is induced by a pulsed laser and detected by a photomultiplier tube. When coupling the high-frequency pulsed laser to the LIF detector we used, the electronics which is designed for continuous wavelength (CW) lasers, “viewed” the laser as a continuous source. Despite this mismatch between the laser and the “ball lens” UV LIF detector, the sensitivity we obtained with tryptophan is comparable to the one obtained with the best “laboratory-made” detector described in the literature which used a CW UV laser. Limits of detection of 0.15 nM for tryptophan and 50 nM for tyrosine were estimated. As an application of this technology, we studied tryptophan and tyrosine in cerebrospinal fluids (CSFs). The analysis is very simple and works on very small samples (5 μl). It consists of using a 10 mM 3-cyclohexylamino-1-propanesulfonic acid, 15 mM sodium tetraborate, pH 9.2 buffer and injecting CSF diluted 20 times in water prior to injection. 5-Hydroxyindoleacetic acid was used as an internal standard. The separation is completed in less than 12 min. The capillary electrophoresis method which we chose is rapid, resolutive and allows accurate measurements. Recovery experiments in CSFs show recoveries between 97 and 102%. We investigated 14 different CSFs from patients who suffered from neurological disorders. Most of the concentrations vary in a range of 1.7 to 3.7 μM for Trp and 6.6 to 13.7 μM for Tyr, which is in the range observed in the literature. One patient who suffers from Huntington disease had a higher concentration of Tyr at 17.3 μM.