The invention concerns a ligand comprising wherein n is an integer from 1 to 5, X represents --NO.sub.2, --NH.sub.2, --NCS, --NHCOCH.sub.2-Z. NHCO--W--COCNHS, --NH-Q, --NHCS-Q, --NHCOCH.sub.2-Q, or --NHCO(CH.sub.2)m ?-Q where Q is an hapten chosen from the group consisting of steroids, enzymes, proteins, monoclonal antibodies, chimeric antibodies, or fragments thereof or any activated linker ready for coupling reaction, Y is CO.sub.2H or PO.sub.3H.sub.2 W is --(CH.sub.2).sub.m-- m is an integer from 1 to 10. Z is chloride, bromide or iodine ##STR00001## ##STR00002##