Small molecule binding by SPR
Drug-like small organic molecules often do not dissolve well in water. Small concentration (<10%) of DMSO is used then to improve drug solubility for SPR analysis. A common technique is to add DMSO solution of drug to the aqueous buffer .
Dielectric properties of DMSO and water are significantly different. As a result, even when the test solution contains no analyte there can be large offset in RU between the reference and test cells. Karlsson et al  have suggested a calibration protocol that eliminates this offset.