Abstract: A fluorometric analyzer includes a head assembly wherein the photomultiplier tube and excitation lamp of the analyzer are received in respective cavities and a transparent test cell is provided to receive a liquid sample. A first optical passage is provided between the lamp cavity and the test cell, and a second optical passage at right angles to the first optical passage is provided between the test cell and the photomultiplier tube cavity. A manually rotatable shutter assembly concentrically disposed about the test cell includes apertures for allowing light to pass through the optical passages when the shutter is rotated to a first position, and for simultaneously blocking the passage of light through the passageways when the shutter is rotated to a second position. Rotation of the shutter is limited by a first guide pin, and a second guide pin provides a desired degree of friction between the shutter assembly and the head assembly housing.