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