Purification of intermediates used in the preparation of heterobifunctional linkers

A method of separating a monocarboxylic acid represented by the following structural formula:

• from a dicarboxylic acid represented by the following structural formula:
• in a mixture comprising the mono and dicarboxylic acids, wherein R1 and R2 are each independently H or a C1-C4 alkyl group, said method comprising the steps of:
• 1) applying the mixture to a silica gel column;
• and
• 2) separating the monocarboxylic acid and the dicarboxylic acid by eluting them from the silica gel column using an elution solvent system and collecting an eluent comprising the monocarboxylic acid, wherein the elution solvent system comprises an organic solvent in the absence of an acid.