Electrospray electrolytic processes

In rare cases, the electrolytic processes during electrospray can cause molecular ion, M" , formation by electrolytic oxidation [95]. This is more probable if nonpolar molecules of very low ionization energy are precluded from other ionization pathways such as protonation or cationization. Electrolytic M" ion formation normally requires nonprotic solvents, low liquid flow for increased reaction time, and preferably metal spray capillaries rather than fused silica capillaries. ESI may also result in changes of the oxidation state of metal ions, e.g., Ag Ag ,... 

Van Berkel, G. J. Zhou, F. Aronson, J. T. Changes in bulk solution pH caused by the inherent controlled-current electrolytic process of an electrospray ion source. Intern. J. Mass Spectrom. Ion Process. 1997, 162, 55-62. 

Van Berkel, G. J. Zhou, E. Observation of gas phase molecular dications formed from neutral organics in solution via the controlled-current electrolytic process inherent to electrospray. J. Am. Soc. Mass Spectrom. 1996, 7, 157-162. 

It can be noted that optimizing a CE-ESI-MS procedure involves a compromise since better electrophoretic efficiencies are obtained in the presence of high ionic strength buffers, whereas the ionization process in electrospray is impaired under these conditions. Finally, even if ESI-MS is a concentration sensitive detector, the dilution of BGE by the sheath liquid does not significantly affect sensitivity since the ionization process occurs in the gas phase and both sheath hquid and buffer electrolyte are completely evaporated. 

For an electrospray to be formed, a minimum concentration of electrolytes, or ions, must be present in the fluid. This is intuitive as the electrospray process involves ions in the fluid and in particular, the movement of these ions by an applied electric field. 

The column effluent from the LC is nebulized into the atmospheric-pressure ion source region. Typical solvents used are mixtures of water and methanol or acetonitrile, containing up to lOmmoll electrolyte, such as formic acid or ammonium acetate. Nebulization can be performed either by means of a strong electrical field, or by a combination of the strong electric field and pneumatic nebulization. The latter is sometimes named pneumatically assisted electrospray or ionspray . The pure electrospray process is limited to flow rates up to 10 pi min Pneumatically assisted electrospray enables the introduction of higher flow rates, up to Imlmin. Since ion production mechanisms between the two modes are identical, the term electrospray is used here throughout. 

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