Embodiments provide a handheld fluorometer and method of determining a concentration of a product within a sample. In some cases the handheld fluorometer includes an immersible sensor head that measures a fluorescence of the product and a controller that calculates the concentration of product. In some cases the handheld fluorometer includes a handheld controller module, an immersible sensor head connected to the controller module, a sample cup for containing a water sample, and a fastener that removably fastens the sample cup about the immersible sensor head. In some cases the sensor head is angled with respect to the controller module and the fluorometer provides a substantially stable base. The sample cup can be removed to acquire a sample of water containing the product and then refastened about the sensor head for determining the concentration.