(2-{Ethyl-[4-(4-nitro-phenylazo)-phenyl]-amino}-ethoxy)-acetic acid-4-nitro-phenyl ester

Incomplete coupling reactions during SPPS can be conveniently detected by treating a sample of the resin with NF31 (the p-nitrophenyl ester of the azo dye (2-{Ethyl-[4-(4-nitrophenylazo)phenyl]amino}ethoxy)acetic acid). The NF31 assay is especially suitable for the detection of sterically hindered primary amines.