Is electrospray emission really due to columbic forces?

Francesco Aliotta,Gabriele Salvato,Cirino Vasi,Mikolaj Pochylski,Rosina C Ponterio,Pietro Calandra

doi:10.1063/1.4894800

Abstract

Electrospray ionization (ESI) is a widely adopted soft ionization method for mass spectroscopy (MS). In spite of the undeniable success of the technique, its mechanisms are difficult to be analytically modelled because the process is characterized by non-equilibrium conditions. The common belief is that the formation of gas-phase ions takes place at the apex of the Taylor cone via electrophoretic charging. The charge balance implies that a conversion of electrons to ions should occur at the metal-liquid interface of the injector needle. We have detected that the above description is based on unproved assumptions which are not consistent with the correct evaluation of the problem. The comparison between experiments performed under the usual geometry and observations obtained under symmetric field configurations suggests that the emitted droplets cannot be significantly charged or, at least, that any possible ionization mechanism is so poorly efficient to ensure that columbic forces cannot play a major role in jet formation, even in cases where the liquid consists of a solution of ionic species. Further work is required to clearly understand how ionization occurs in ESI-MS.

Highlights

Mass spectroscopy (MS) based on electrospray ionization (ESI) has been widely adopted over more than thirty years
The comparison between experiments performed under the usual geometry and observations obtained under symmetric field configurations suggests that the emitted droplets cannot be significantly charged or, at least, that any possible ionization mechanism is so poorly efficient to ensure that columbic forces cannot play a major role in jet formation, even in cases where the liquid consists of a solution of ionic species
Summarizing our results, we have detected that, contrary to the current belief, no net charge is necessary to explain the observed motion of the drops in a usual electrospray experiment

Summary

Introduction

Mass spectroscopy (MS) based on electrospray ionization (ESI) has been widely adopted over more than thirty years. In the typical experimental set-up the liquid is flowing through a polarized needle, the injector, while a second electrode, the extractor, is a large plate located few centimeters away. It has been argued[3] that dielectrophoretic forces acting on liquid droplets cannot explain jet emission. The movement of droplets always occurs from the needle towards the large extractor plate This observation has been enough to deduce that the flying liquid drops must possess a net charge of the same sign as the needle electrode.[4,5] Partial corona discharge[6] at the needle could cause necessary ionization. Jet disintegration has been usually explained in terms of columbic explosion occurring when, due to solvent evaporation, the net charge existing in each droplet exceeds the Rayleigh limit.[7,8,9] This is the current explanation of the electrospray mechanism adopted in the literature.[10]

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Conclusion