Investigation of reduction of Cu(II) complexes in positive-ion mode electrospray mass spectrometry

The electrospray ionization mass spectrometry (ESI-MS) behavior of seven Cu (II) complexes with tetradentate ligands has been studied. An unexpected re duction process, in positive ion mode, of the Cu oxidation state was observ ed, and shown to be due to charge transfer between the metal complex and th e solvent molecules in the gas phase. Ion trap collision-induced dissociati on experiments and deuterated solvents were used to support the proposed me chanism that is not a common electrochemical redox reaction at the ESI tip, but a gas-phase process. A series of solvents (acetonitrile, methanol, eth anol, propanol and iso-butanol) were tested, and a correlation between ioni zation energy (IE) and the amount of Cu(I) produced in EST has been demonst rated for the alcohols, although some other solvent properties should also be taken into account. The electrochemical reduction potential of the compl exes in solution is also an important parameter, since complexes more easil y reduced in solution are also easier to reduce in the gas phase. Copyright (C) 2001 John Wiley & Sons, Ltd.