Chromatography Forum

The standard mobile phase additives for ESI negative seem to be either NH4OAc or NH4OH. Some publications, however, suggest that acetic conditions is not only favourbale for the formation of cations but also for the formation of anions in the electrospray process. The suggested reasoning behind it is the following:

"To accomplish successful ESI analysis, it is imperative to obtain excess charge on the droplets through an electrochemical reaction that occurs at the spray tip. In negative-ion mode, the dominant reaction is reduction. Within unmodified solutions of acetonitrile and water, protons arising from water or the analytes are presumably reduced to form hydrogen gas. The additional protons provided by an acidic modifier facilitate reduction, making it easier for the spray droplets to carry excess negative charge. This excess negative charge likely accumulates on the surface of the droplet due to electrical repulsion during negative-ion ESI, increasing the pH on the surface of the droplet and providing a local environment in which deprotonation of the analytes occurs more easily than would occur in the bulk solution."

What do you think about it?
What is your personal experience?


Source: "Favorable Effects of Weak Acids on Negative-Ion Electrospray Ionization Mass Spectrometry"

I can't prove or disprove the mechanism of what's actually happening on the surface of electrospray droplets, but I have watched a large number of chemists over the years (re)discover that changing to a basic buffer, or doing post-column additions to increase the pH, generally reduces the efficiency of ionisation in negative mode rather than increasing it. Electrospray is clearly magic.
Subject to avoiding solvent additives that would damage the instrument, I always advise people to choose the buffer and pH that give them the best chromatography, and let the electrospray take care of itself.