An optical apparatus with a single-use, disposable fluid flow cartridge and cell and associated optical interface is employed in determining characteristics of a fluid and/or suspended materials or cells contained therein which are introduced into the apparatus. The optical interface communicates electromagnetic radiation from an optical instrument through the fluid within the cell and to an appropriate sensor within the optical instrument. The interaction of the electromagnetic radiation with the fluid is measured by the signal generated by the sensor. Fluid present within the cell may be static or flowing, allowing both discrete sample measurements and monitoring of continuous processes.