High production of small organic dicarboxylate dianions by DESI and ESI.

A significant production of gas-phase dicarboxylate dianions has been observed in standard ESI and DESI during the analysis of small organic dicarboxylic acids under moderate or highly alkaline conditions. In ESI, this can be attributed to an excess of hydroxyl ions (OH(-)) which favor the formation of an high amount of dianions in solution, contemporarily trapping the potential counter ions during the ESI process. The results obtained in DESI highlight the role of the surface in trapping the counter-ions during desorption process and determining the ultimate nature of the observed gas-phase ions.