The invention concerns a ligand comprising wherein n is an integer from 1 to 5, X represents —NO2, —NH2, —NCS, —NHCOCH2-Z. NHCO—W—COCNHS, —NH-Q, —NHCS-Q, —NHCOCH2-Q, or —NHCO(CH2)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 CO2H or PO3H2 W is —(CH2)m— m is an integer from 1 to 10. Z is chloride, bromide or iodine