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The electrospray ionization mass spectrometry (ESI-MS) behavior of seven Cu(II) complexes with tetradentate ligands has been studied. An unexpected reduction process, in positive ion mode, of the Cu oxidation state was observed, and shown to be due to charge transfer between the metal complex and the solvent molecules in the gas phase. Ion trap collision-induced dissociation experiments and deuterated solvents were used to support the proposed mechanism that is not a common electrochemical redox reaction at the ESI tip, but a gas-phase process. A series of solvents (acetonitrile, methanol, ethanol, propanol and iso-butanol) were tested, and a correlation between ionization energy (IE) and the amount of Cu(I) produced in ESI has been demonstrated for the alcohols, although some other solvent properties should also be taken into account. The electrochemical reduction potential of the complexes in solution is also an important parameter, since complexes more easily reduced in solution are also easier to reduce in the gas phase. Copyright Electrospray ionization mass spectrometry (ESI-MS) has emerged as an important technique for the analysis of a wide range of molecules. Although its major application is in the biochemistry field, the capacity of ESI to deal with inorganic and organometallic complexes was explored very soon after its introduction. 1Electrospray is used extensively nowadays in the field of metal ion complexes because of its capability to produce gasphase ions that are strongly correlated to the solution structure.2 Despite this strong correlation, sometimes structures and characteristics of metal complexes can be modified during the electrospray process, and rearrangements can occur when transferring ions from solution to the gas phase. Electrospray ionization of neutral or non-polar compounds is less efficient than that of polar compounds, and sometimes can produce molecular radical cations through an electrochemical oxidation process in the ESI needle at the liquid-metal interface. Redox reactions have been reported to occur in ESI-MS; 5 either electrochemical oxidation or reduction can be observed depending on the high voltage polarity applied to the ESI capillary (i.e., positive or negative mode). An electrospray source can be described as ªan electrolytic cell of a somewhat special kindº. 6 These redox reactions are essential to maintain the charge imbalance inside the formed droplets, although the importance of such electrochemical processes in understanding ESI and its analytical significance remains a much argued question, see, e.g., de la Mora et al. 7In transferring ions from solution to the gas phase, additional changes and rearrangements can also occur since ions are transferred into a very different environment. New conditions can induce ion structure transformation. For example, multiply charged ions, in which the charges are localized on one atom or a small group of atoms (e.g. multiply charged metal ions, M z ), because of the high columbic repulsion, are difficult to observe in the...

Seven isomeric hydrocarbons were detected in the volatile fraction of three samples of virgin olive oil. Chemical ionization mass spectrometry was used to assign the molecular formulas. Their structures were confirmed by comparison of retention time and mass spectral data with those of a synthetic sample obtained by pentene radical coupling. A final characterization of each chromatographic peak was done by means of a chiral capillary column to distinguish the optically active compounds from the isomers without chiral centers. For the quantitation the recovery factor in the oily matrix during the extraction of the volatile fraction was obtained using a related chemical, the commercially available beta-citronellene. On the basis of the previous literature and the present experiments a tentative rationalization of the involved biochemical pathway is proposed.

The applicability of a trypsin-based monolithic bioreactor coupled on-line with LC/MS/MS for rapid proteolytic digestion and protein identification is here described. Dilute samples are passed through the bioreactor for generation of proteolytic fragments in less than 10 min. After digestion and peptide separation, electrospray ionization tandem mass spectrometry is used to generate a peptide map and to identify proteolytic peptides by correlating their fragmentation spectra with amino acid sequences from a protein database. By digesting picomoles of proteins sufficient data from ESI and MS/MS were obtained to unambiguously identify proteins alone and in serum samples. This approach was also extended to locate mutation sites in beta-lactoglobulin A and B variants.