The present invention relates to the field of radiopharmaceuticals for in vivo imaging, in particular to a method of labelling a biological targeting molecule with the radioisotope 18F. The invention provides a method of preparation of lyophilised compositions of aminooxy-functionalised biomolecules, as well as radiolabelling methods using the purified materials. Also provided are lyophilised compositions and cassettes comprising such purified compositions. The invention is particularly suitable for use with an automated synthesizer apparatus.