Competitive binding of aqueous perfluoroctanesulfonic acid and ibuprofen with bovine serum albumin studied by electrospray ionization mass spectrometry

► PFOSX, at a concentration half that of ibuprofen, outcompetes ibuprofen in binding to BSA. ► PFOSX bound to BSA prevents ibuprofen from binding to the protein. ► A maximum of nine PFOSX molecules were shown to bind to BSA. ► Only two molecules of ibuprofen were demonstrated to bind with BSA in its native state.