The present invention relates to the preparation of a metal chelating separation medium, which comprises providing a compound defined by formula: R--O--CO--(CH.sub.2).sub.n--CHNH.sub.2--CO--OH wherein R is a selectively cleavable group; and n is an integer of 1 4; reacting it with ethyl alcohol; derivatisation; cleavage of R; immobilisation of the resulting compound to a base matrix; and deprotection of the carboxylic acid. In one embodiment, the compound provided in the first step is ethyl ester of aspartic acid or of glutamic acid, and the derivatisation agent is bromo-acetic acid ethyl acetate.