Reversed-phase LC determination of two manufacturing intermediates, the unsulfonated subsidiary color, and 4-methyl-Sudan I in D&C Red No. 6, D&C Red No. 7, and their lakes.

A reversed-phase liquid chromatographic (LC) method was developed to determine 4 impurities in the monosulfo monoazo color additives D&C Red No. 6, D&C Red No. 7, and their lakes. Manufacturing intermediates determined are 2-amino-5-methylbenzenesulfonic acid (PTMS) and 3-hydroxy-2-naphthalenecarboxylic acid (3-hydroxy-2-naphthoic acid). Subsidiary colors determined are the unsulfonated and uncarboxylated, unsulfonated homologs of the dye (the unsulfonated subsidiary color and 4-methyl-Sudan I). The new method uses lithium oxalate as part of a novel system to dissolve the water-insoluble color additives. The 4 impurities were identified by LC and UV-visible spectrophotometry; the 4-methyl-Sudan I was confirmed by LC/mass spectrometry. LC peak area calibrations were generally linear (R > 0.999), and recoveries ranged from 90 to 105%. The limits of determination used were 0.02% each for PTMS and 3-hydroxy-2-naphthoic acid, 0.1% for the unsulfonated subsidiary color, and 0.002% for 4-methyl-Sudan I. The relative standard deviations at the specification levels ranged from 0.7 to 22.7%. Survey analyses of 26 samples of certified D&C Red Nos. 6 and 7 straight colors and lakes from 11 domestic and foreign manufacturers yielded results that agree with those previously obtained using 3 other methods.