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 (CH3 CN, tetrahydrofuran, CH3 OH, H2 O) 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.