No abstract

The relationship between one‐electron (e−) oxidation processes and the formation of radical cations of endogenous and exogenous compounds in vivo is of considerable interest. This paper reports on the experiments that allow FTICR mass spectrometric (MS) detection of ion signals that are consistent with the formation of radical cations of caffeine (CA) and theophylline (TP) during electrospray ionization (ESI) in ESI FTICR MS and in on‐line electrochemistry (EC)/ESI FTICR MS in positive mode. Significantly, the signals of the radicals of CA•+ and TP•+can be enhanced by simple modifications of the operating conditions in ESI MS, facilitating investigations of radical formation and related reactions.

Electrolytic efficiency of battery‐operated flow‐through thin‐layer electrochemical (EC; echem) cell used in EC/ESI‐MS was investigated. Two stainless steel capillaries joined by PTFE tubing formed the thin‐layer EC cell. Echem reaction efficiency of ferrocyanide, dopamine and hydroquinone oxidation in the EC cell was low and decreased with increasing MeOH volume fraction in the MeOH/H2O carrier solutions with low conductivity. Water oxidation dominated the cell current and migration of ferrocyanide was observed. Flow rate of 50–200 µL/h had a very small effect on EC cell current. Dopamine oxidation current in the EC cell and dopamine ion intensity in EC/ESI‐MS showed a direct correlation.

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 DOPADA , 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.