Stability of 5,5-dimethyl-1-pyrroline-N-oxide as a spin-trap for quantiﬁcation of hydroxyl radicals in processes based on Fenton reaction /J.M. Fontmorin a, * , R.C. Burgos Castillo a , W.Z. Tang b , M. Sillanp€a€a a - Fontmorin,J.M.