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In order to overcome a common problem in spin trapping with high concentrations of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) where ESR spectra are obtained which include an unidentified set of lines composed of a triplet of doublets, commercial DMPO was analyzed for its impurities by high-performance liquid chromatography, mass spectrometry, and nuclear magnetic resonance spectroscopy. It has been determined that this undesirable ESR spectrum is due to an impurity included in the spin trap. This compound has been assigned to the hydroxylamine which is a DMPO-derivative having an epoxy ring located at the 2 and 3 positions.

Aminoxyl radical formation from DMPO in the presence of Fe ion was studied to clarify the ambiguous ESR signals obtained by spin trapping with DMPO. It was found that when DMPO was used in a Fenton system, a Fe-DMPO complex was formed immediately. This complex was subsequently attacked by oxidative species originating from H2O2 and thus oxidative degradation of DMPO was induced in the Fenton system. On the other hand, in the case of M4PO, the degradation was found to be very slow, indicating that the 3 position of DMPO was favorably attacked by the oxidative species. Some of the degradation products are probably aminoxyl radicals. This series of the degradation products are probably aminoxyl radicals. This series of reactions may compete with spin trapping and make it difficult to analyze ESR spectra obtained in the presence of Fe ion.