A covalently reactive ligand analogue (CAL) of formula (1): wherein, L1 . . . Lx . . . Lm are components defining a ligand determinant, Lx is a component unit of the ligand determinant selected from the group consisting of an amino acid residue, sugar residue, a fatty acid residue and a nucleotide, L′ is a functional group of Lx, Y″ is atom, covalent bond or linker, Y′ is an optional charged or neutral group Y is a covalently reactive electrophilic group that reacts specifically with a receptor that binds to said ligand determinant, and n is an integer from 1 to 1000 m is an integer from 1 to 30.