DOPA and Adrenaline Oxidation Kinetics and Intermediates Identified by Electrospray Ionization Mass Spectrometry in Real Time

DOPA and adrenaline (Adr) oxidation intermediates were identified using electrospray ionization mass spectrometry (ESI MS) by exploiting electrochemical oxidation reactions that occur during ESI. In addition, on‐line MS monitoring of the oxidations allowed evaluation of DOPA and Adr oxidation kinetics with approximately 3 ms time resolution. Kinetic information was confirmed by on‐line electrochemistry/ESI MS (EC/ESI MS). Application of an EC cell voltage in EC/ESI MS increased the oxidation rates and MS detection sensitivity of intermediates. During ESI and EC/ESI MS, oxidation rates were DOPA < Adr ≪ dopamine ( DA ) , cyclization rates of quinone oxidation products were Adr ≫ DOPA > DA , and cyclic leucochrome oxidation rates were DA>Adr>DOPA. Formation of radical semiquinones of Adr and DOPA through OH . radical‐initiated oxidation pathways was indicated, as reported for DA. Images of catecholamine oxidation pathways obtained with high time resolution, using MS, underscore the differences between the reactivity of DOPA, Adr, and DA.