Anhydride of what? Formic anhydride does not exist, and acetic anhydride will form the N-acetyl derivative of tryptamine and N-alkyltryptamines, and leaving dialkyltryptamines alone - after this treatment only dialkyltryptamines will carry through an acid/base extraction while less substituted tryptamines will be converted to acetyl-amides and can be removed from the product amine easily.