Determination of p-cresidine in FD&C red no. 40 by the diazotization and coupling procedure followed by reversed-phase high-performance liquid chromatography.

The free aromatic amine p-cresidine is determined at low ppb (10(9] levels in the regulated color additive FD&C Red No. 40. The determination involves chloroform extraction of p-cresidine and other unsulfonated impurities from the color additive, followed by diazotization of the aromatic amines and coupling with the disodium salt of 3-hydroxy-2,7-naphthalenedisulfonic acid (R-salt). The coupling products are then determined by reversed-phase high-performance liquid chromatography. Calibration is performed in the presence of the color additive by using the external standard method. The identity of the p-cresidine coupling product is confirmed by obtaining a UV-VIS spectrum of the eluting solute. The presence of aniline in commercial batches is also confirmed in this manner by using 4,5-dihydro-5-oxo-1-(4-sulfophenyl)-1H-pyrazole-3-carboxylic acid (pyrazolone-T) as an alternative coupling agent.