Surface plasmon resonance (SPR) is an assay technique where the target molecule is immobilised on a slide. Light is refracted off the slide, and the refraction angle recorded. The drug molecule is washed over the slide and light refracted again. The change in refraction angle can be used to determine the molecular weight of the drug, and the change that has occurred.
This process is frequently used in pharmaceutical drug-binding assays, but does not always represent the real-world.