ESI activity of Br−, BF4−, ClO4− and BPh4− anions in the presence of Li+ and NBu4+ counter‐ions

To improve our understanding of the electrospray ionization (ESI) process, we have subjected equimolar mixtures of salts A⁺X^? (A⁺ = Li⁺, NBu₄⁺; X^? = Br^?, ClO₄^?, BF₄^?, BPh₄^?) in different solvents (CH₃CN, tetrahydrofuran, CH₃OH, H₂O) to negative‐ion mode ESI and analyzed the relative ESI activity of the different anionic model analytes. The ESI activity of the large and hydrophobic BPh₄^? ion greatly exceeds that of the smaller and more hydrophilic anions Br^?, ClO₄^? and BF₄^?, which we ascribe to its higher surface activity. Moreover, the ESI activity of the anions is modulated by the action of the counter‐ions and their different tendency toward ion pairing. The tendency toward ion pairing can be reduced by the addition of the chelating ligands 12‐crown‐4 and 2.2.1 cryptand and is, although to a smaller degree, further influenced by the variation of the solvent. Complementary electrical conductivity measurements afford additional information on the interactions of the ionic constituents of the sample solutions. Copyright © 2017 John Wiley & Sons, Ltd.