A typical SPRi instrument configuration. A gold film is coated onto one face of a glass prism, then prepared with a microarray of different capture “probes”, and finally exposed to the sample solution. The binding of analytes to the surface changes the local refractive index (RI), which is detected via a corresponding change in the backside reflectivity of the film by a p-polarized laser beam. The illuminating beam is configured to excite surface plasmon polaritons in the gold film, which are highly sensitive to changes in local RI. Adapted from .