Sequential counting of single molecules in a microcapillary

We studied photon bursts of individual rhodamine 6G molecules in ethylene glycol flowing through a microcapillary (diameter 1 mu m) Excitation was provided by an argon ion laser (lambda = 514.5 nm). The laser beam was coupled into a confocal microscope by a glass plate and focussed onto the sample by an oil-immersion objective (magnification 100x). Fluorescence was collected by the same objective and detected by a photomultiplier. In order to reject out-of-focus signals a pinhole (diameter 100 mu m) was placed in the image plane. Several filters were used to suppress laser stray light and Raman scattered photons. With the setup described sequential counting of individual molecules is possible.