Prototype fluorimeter for the XLA/XLI analytical ultracentrifuge

A fluorescence detector has been developed for the XLA/XLI analytical ultracentrifuge. The excitation source consists of an Argon ion laser coupled to a single-mode, 3.5 micron diameter fiber. The remaining optics are in the rotor chamber and attached to a 500 steps/second, 2 microns/step motorized stage mounted above the centrifuge rotor. The excitation and emission paths are coaxial. The excitation spot is about 7 microns in diameter and is focused about 1 mna below the top window of a centrifuge cell. Radial resolution is achieved by scanning this spot along a radial axis. Emitted light is gathered by the condensing lens and redirected by the beam splitter to a condensing lens focused on a pinhole. A long-pass filter sits between the pinhole and a photomultiplier tube. Operation of the stepping motor and photomultiplier electronics is under computer control, allowing data to be acquired from a radial stripe around the rotor after each step. This data acquisition system will permit the monitoring of specifically labeled compounds in the presence of a large background of other molecules. Both the sensitivity and the selectivity of this optical system will be useful for a range of biological problems.