You may be able to use vacuum distillation if the compounds are somewhat volatile and heat stable.

What are the impurities? Depending on what those are, you may be able to use other purification methods. As an example, you may be able to partition your mixture with basic water. Make the aqueous layer acidic, then extract the free acis into an organic solvent; this will remove the neutral and basic impurities.

Anothe possiblity is ion exchange resins.

Thanx a lot Jack. This is very useful for me.