Mapping of Potential Gradients within the Electrospray Emitter

A novel electrochemical probe has been designed, built, and used to characterize the distribution in solution potential within the metal capillary and Taylor cone of the electrospray (ES) device. The measurement system consists of three electrodes-a counter electrode held at highly negative potential that serves as the cathode, and two anodes consisting of a disk-shaped, mobile, internal (working) electrode, and the internal surface of the surrounding ES capillary (auxiliary electrode, held at ground potential). One-dimensional differential electrospray emitter potential (DEEP) maps detailing solution potential gradients within the electrospray emitter and in the region of the Taylor cone are constructed by measuring the potential at the working electrode vs the ES capillary, as a function of working electrode position along the emitter axis. Results show that the measured potential difference increases as the internal probe travels toward the ES capillary exit, with values rising sharply as the base of the Taylor cone is penetrated. Higher conductivity solutions exhibit potentials of higher magnitude at longer distances away from the counter electrode, but these same solutions show lower potentials near the ES capillary exit. Removal of easily oxidizable species from the solution causes the measured potential difference to have nonzero values at distances further within the capillary, and the values measured at all points are raised. Results are consistent with the characterization of the electrospray system as a controlled-current electrolytic flow cell. Elucidation of the electrochemical details of the electrospray process can lead to mass spectrometric signal enhancement of certain species present in the spraying liquid and also allow the detection of molecules that are usually not observable due to their low ionization efficiencies.