Angular distribution of fluorescence from dye-filled capillaries

We studied the angular distribution of fluorescence from a small, lossy capillary filled with a laser-dye solution. We found that the fluorescence is isotropic for the liquid core and that, far from the liquid-solid phase transition, this isotropy shows no temperature dependence. This result, an extension of studies with solid cylinders, is at variance with theoretical expectations for solids as well as with previous reports by other investigators but is explained by the motion of the molecules in the liquid. Therefore the optimal viewing angle for capillary zone electrophoresis experiments is near 90° because the elastic scattering of the incident laser light is at or near a minimum for these small capillaries. This reduces contamination of the fluorescence signal as a result of stray laser light in the optical system.