Redox buffering in an electrospray ion source using a copper capillary emitter

An electrospray ion source used in electrospray mass spectrometry is a two- electrode, controlled-current electrochemical flow cell. Electrochemical re actions at the emitter electrode (oxidation and reduction in positive and n egative ion modes respectively) provide the excess charge necessary for the quasi-continuous production of charged droplets and ultimately gas-phase i ons with this device. We demonstrate here that a copper capillary emitter, in place of the more commonly used stainless-steel capillary emitter, can b e utilized as a redox buffer in positive ion mode. Anodic corrosion of the copper capillary during normal operation liberates copper ions to solution and in so doing maintains the interfacial potential at this electrode near the equilibrium potential for the copper corrosion process [E degrees = 0.3 4 V versus standard hydrogen electrode (SHE)]. Fixing the interfacial poten tial at the emitter electrode provides control over the electrochemical rea ctions that take place at this electrode. It is shown that the oxidation of N-phenyl-1,4-phenylenediamine to N-phenyl-1,4-phenylenediimine (E-p/2 = 0. 48 V versus SHE) can be completely avoided using the copper emitter, wherea s this analyte is completely oxidized with a stainless-steel capillary emit ter under the same conditions. Moreover, using N-phenyl-1,4-phenylenediimin e, we demonstrate that reduction reactions can occur at the copper emitter electrode in positive ion mode. Emitter corrosion, in addition to redox buf fering, provides a convenient means to introduce metal ions into solution f or analytical use in electrospray mass spectrometry. Published in 2001 by J ohn Wiley & Sons, Ltd.