Electrospray as an Electrolytic Cell

The other alternatives for restoring the charge balance are the removal of negative ions present in the solution by an oxidation reaction as illustrated in Eq. (1.3) 

Van Berkel and coworkers have examined the consequences of the electrochemical processes to ESI-MS in a series of publications [17]. They were able to demonstrate that ions produced by the electrolysis process can in some cases have unintended and undesired effects on the mass spectra obtained with pH- or oxidation-sensitive analytes [17]. 

Ions introduced into the solution by inadvertent or deliberate electrolysis amount to very low concentrations. Taking the Zn capillary tip as example, a solution of 10 M NaCl in methanol at a flow rate Vf=20p,Lmin was found to lead to an electrospray current of 1.6 x 10 A. The Zn concentration produced by the Zn-tipped capillary evaluated from the current was 2.2 x 10 M. Assuming that the Na ion was the analyte ion, the concentration of the ions produced by the oxidation at the electrode is only 1 /5 of that of the analyte. It will be shown later that the electrospray current increases very slowly with the total electrolyte concentration. Therefore, ions produced by oxidation at the electrode may not be noticed in the mass spectrum at higher analyte or additive concentrations. 

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