Abstract: High purity water, particularly that intended for the pharmaceutical or electronics industry, is analyzed for the presence of pyrogen or other impurities by causing the water to come into contact with a direct affinity sensor, which may be a surface plasmon resonance (SPR) device or other sensor relying on an evanescent wave phenomenon. A property of the surface—refractive index in the case of SPR—changes on the binding of impurity, thereby enabling impurity to be detected. The invention overcomes the cumbersome nature and batch-to-batch variability of the conventional in vivo tests as well as the in vitro Limulus Amoebocyte Lysate (LAL) assay and for the first time allows the continuous or real time monitoring of high purity water for pyrogen.