text: Another identification mechanism is through immunoassay. Abbott Laboratories AxSYM is an immunoassay device utilizing Fluorescence Polarization Immunoassay (FPIA) technology that can determine the presence and quantify salicylates. The introduction of a salicylate specific antigen labeled with fluorescein into the sample will mark the sample.  Upon irradiation with 490nm light, some of that light will be reflected back to a detector at 520nm.  Polarization allows the machine to detect the difference between antibody bound, and unbound fluorescein. It is therefore possible to quantify the serum salicylate level through the signal strength—the amount of reflected light received.