[Determination of the key impurity in caprolactam produced from a combined process by gas chromatography-mass spectroscopy].

Caprolactam (CPL) is an important chemical raw material. Because even trace level contaminations in CPL can directly influence the polymerization and the resulting properties of fibers, the CPL monomer must be very pure. In this study, gas chromatography with a flame ionization detector (GC/FID) and gas chromatography mass spectroscopy (GC-MS) techniques were developed to determinate the key impurities in CPL using a novel process which characterized the ultraviolet (UV) absorption rates of the samples. The GC-MS results indicated that the major impurity had a relative molecular mass of 188 and was qualitatively characterized as 1,2,3,4,6,7,8,9-octahydrophenazine. Considering the reaction conditions and properties of the substances in the reactor, we speculated that the octahydrophenazine may have originated from a Neber rearrangement, which is a side-reaction of the Beckmann rearrangement. The impurity contains strong chromophores and severely impacted the UV absorption rate of the CPL product. Therefore, during CPL production, the concentration of octahydrophenazine must be strictly controlled.