Differentiating between capillary and counter electrode processes during electrospray ionization by opening the short circuit at the collector

The chemical reactivity of an electroactive neutral compound in the conditions of electrospray mass spectrometry was studied by carrying out preparative experiments, The residue obtained by spraying a micromolar methanolic solution of 9-nitroanthracene onto a metallic plate in negative mode (cathodic capillary) was collected and then analyzed by GC/MS, showing that both reduction and oxidation products were formed. To distinguish between the two processes,the two functions of the metallic plate, definition of the electric potential and storage of products, were separated by using a copper-bronze grid as counter electrode and a silica TLC plate, located a few millimeters behind, as collector. The collected products were thus stored away from the counter electrode, avoiding their possible reoxidation at this site. The amount of anthracene, a reduction product of 9-nitroanthracene, which was detected only in trace amounts with the metallic plate collector, increased significantly with the new device. Thus, anthracene can be viewed as the result of a capillary process whereas 9,10-anthraquinone can be estimated as mainly due to further oxidation at the collector. Furthermore, in deuterated solvents no incorporation was detected, suggesting that some reaction steps take place at the collector.