Abstract: The present invention can be described as an apparatus and cartridge for detecting the presence of an analyte in a fluid. The invention can also be described as a method of detecting the presence of an analyte in a fluid using the apparatus and cartridge. The method comprises the steps of clamping a sensor cartridge into a cartridge receiver of the apparatus, providing fluid to a fluid flow-path of the cartridge, illuminating at least a portion of a sensor, and using a photodetector to detect a change in the optical property of the sensor, wherein the change is caused by the presence or absence of an analyte in the fluid.