Many suitable chelants are widely known or have been described in the literature, especially literature relating to heavy metal detoxification agents bifunctional chelants and chelate-based contrast agents, e.g. those described in WO-A-89/00557 (Berg) and the documents mentioned therein and in the search report appended thereto, U.S. Pat. No. 4,647,447 (Gries), U.S. Pat. No. 4,826,673 (Dean), EP-A