An equilibrium partitioning model for predicting response to host-guest complexation in electrospray ionization mass spectrometry.

While electrospray ionization mass spectrometry (ESI-MS) has become a powerful technique for analyzing many types of host-guest complexation, questions remain as to just how accurately ion abundances generated by ESI reflect the true distribution of species at equilibrium in solution. To better understand this relationship, an equilibrium partitioning model was developed to explain the various interactions that dictate how much of a particular host-guest complex is transferred from solution into the gas phase in the ESI process. By evaluating the simultaneous equilibria of the complexation reaction and the partitioning of species between the surface and interior of the ESI droplets, one can estimate the ion abundances generated. The predictions of this new model were evaluated and experimentally confirmed through the analysis of the complexes of 18-crown-6 with alkali metal cations in an ESI quadrupole ion trap mass spectrometer, and it was determined that binding constants alone may not give accurate predictions about the observed ESI-MS response to different host-guest complexes.