Abstract: The compositions of impurities in the concentrated mother liquids after the recrystallizations of three various quality grades caprolactam products were analyzed by GC-MS to compare the impurities that affect the volatile base contents in these products. Results showed that the impurities were mainly composed of C3-C7 amides with straight chain, C6 imides, oximes and aniline, in which C3-C7 amides with straight chain and C6 imides had significant effect on the volatile base content: with the increase of 0.1 mmol/kg these impurities, the volatile base content could increase 0.1 mmol/kg. Aniline had less effect on the volatile base content; oximes and lactams had little effect. As compared with the superior caprolactam product, in the qualified product impurities were mainly composed of C3-C7 amides with straight chain and C6 imides, in the first-class product minor impurities were n-valeramide and n-caproamide, however, the existence of these impurities in products leads to a higher volatile base content, which degrades the product class.

Key words: caprolactam, volatile base, re-crystallization, Product quality