Abstract: | The fragmental behavior of some monocyclic N‐aryl‐δ‐valerolactams in EI‐MS was studied. Their molecular ion peak, together with some characteristic fragments such as M‐29]+, M‐56]+?, M‐69]+, and M‐98]+, were always found in a series of N‐aryl‐δ‐valerolactams in EI‐MS spectra. Furthermore, the mechanism for the interpretation of each fragment is described. |