CHARACTERIZATION OF AN ELECTROSPRAY ION-SOURCE AS A CONTROLLED-CURRENT ELECTROLYTIC CELL

An electrospray (ES) ion source is described as a constant or controlled-current device for which the magnitude of the ES current is controlled by the rate of charged droplet production. Thus, the nature of the electrolytic process that occurs in the metal ES capillary to charge balance the loss of one ion polarity in the charged droplets is shown to be analogous to that of a controlled-current electrolytic (CCE) cell and controlled-current electrolysis carried out in a now cell. That is, the potential at the metal/solution interface in the ES capillary, which ultimately determines whether or nota particular species will undergo a redox reaction in the capillary, is a function of both the ES current and the relative redox potentials and concentrations of the various species in the solvent system, including the metal capillary. Furthermore, the extent to which one or more reactions occur is limited both by the ES current and by the now rate of the solvent system through the ES capillary. Experimental confirmation of the ES ion source as a CCE cell is made through experiments employing a novel ES ion source in which the effluent from the ES capillary enters the detection cell of a UV/visible diode array spectrophotometer prior to the spraying process. This ES setup allowed for the first time the detection of the products of the redox reactions in the ES capillary, while they were still in solution, thereby avoiding experimental complications imposed by the spraying process or by the subsequent mass analysis of the gas-phase ions that might complicate data interpretation. The analytical implications of the operation of the ES ion source as a CCE cell for neutral compound ionization and detection in ES-MS are briefly discussed.