Laser-induced fluorescence spectrometry is used to profile pH changes as droplets evaporate in an electrospray plume by measuring emission spectra of 2(or 4)-[10-(dimethylamino)-3-oxo-3H-benzo[c]xanthene-7-yl]-benzenedicarboxylic acid (carboxy SNARF-1), a pH-sensitive fluorescent dye. The observed pH changes depend on initial droplet pH and polarity. In some instances, small or negligible changes of pH are observed, consistent with expected buffering. The pH of initially acidic droplets decreases along the spray axis in both positive and negative ion modes, to an extent larger than expected from solvent evaporation. This phenomenon may be a manifestation of droplet cooling, droplet subdivision, or heterogeneous charge distribution within the spray plume or within individual ES droplets.
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Zhou et al. (2002) studied this question.
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