Geometry induced fluorescence modifications: An experimental study of micron-size capillaries.

Abstract

Capillary Zone Electrophoresis (CZE) is an important analytical technique used to analyze the chemical makeup of small samples with volumes on the order of picoliters. However, in the quest for smaller sample volumes, smaller capillaries are used. This research has focussed on analyzing the optical properties of these small capillaries to determine if and how they affect the fluorescence spectrum of the molecules inside the capillary. In the course of this research we have determined that for fluorescent liquids, there is no evidence of angular dependencies in the fluorescent emissions for the total intensity or the linear polarization components of the total intensity. This angular isotropy does not show any noticeable temperature dependence. We have also determined that the fluorescence spectrum from these molecules is not smooth. Even with weak reflections from the capillary inner walls, enough feedback exists to superimpose resonances on the otherwise smooth spectrum. If the reflectivity is high enough, typically when the core index of refraction is higher than the capillary's index, it is possible to cause some materials to lase. This lasing has been shown in a variety of systems by several researchers. However, total internal reflection is not required for resonances, although weak, to be present. Resonances between the cavity dimensions and the incident laser light are also responsible for a nonlinearity in fluorescence intensity normalized to volume. This normalized intensity nonlinearity may also be due partially to photoinduced decay processes that were observed. In any event, some care is required in the selection of capillary size for CZE in order to maximize fluorescence intensity per unit volume from the capillary. The use of small capillaries exposes the experimenter to a wide variety of physical phenomena which make their use and appropriate interpretation of the data somewhat complex, although manageable.