The Mechanism of Electrospray Ionization

Electrospray ionization occurs by the same four steps as listed above for thermospray (see Section 4.6). In contrast to thermospray, and most other ionization methods used in mass spectrometry, it should be noted that electrospray ionization unusually takes place at atmospheric pressure. A similar process carried out under vacuum is known as electrohydrodynamic ionization and gives rise to quite different analytical results. This technique has not been developed into a commercial LC-MS interface and will not be considered further. 

A series of such explosions then takes place until a point is reached at which ions of the appropriate analytes dissolved in these droplets are produced and are transferred through a series of focusing devices (lenses) into the mass spectrometer. 

There are alternative explanations for the actual mechanism by which these ions are produced, e.g. the ion-evaporation [11] and charge-residue models [12], and these have been debated for some time. 

According to the ion-evaporation model, the droplets become smaller until a point is reached at which the surface charge is sufficiently high for direct ion evaporation into the gas phase to occur. In the case of the charge-residue model, repeated Coulombic explosions take place until droplets are formed that contain a single ion. Evaporation of the solvent continues until an ion is formed in the vapour phase. 

The lack of a definitive explanation, however, does not affect our ability to appreciate the analytical capabilities of the technique, the HPLC characteristics that will affect the production of ions by the electrospray process and the mass spectra that may be obtained. 

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We have previously considered the mechanism of electrospray ionization in terms of the charging of droplets containing analyte and the formation of ions as the charge density on the surface of the droplet increases as desolvation progresses. The electrospray system can also be considered as an electrochemical cell in which, in positive-ion mode, an oxidation reaction occurs at the capillary tip and a reduction reaction at the counter electrode (the opposite occurs during the production of negative ions). This allows us to obtain electrospray spectra from some analytes which are not ionized in solution and would otherwise not be amenable to study. In general terms, the compounds that may be studied are therefore as follows ... 

Kebarle, P Tang, L. From Ions in Solution to Ions in the Gas Phase The Mechanism of Electrospray Ionization Mass Spectrometry, Anal. Chem. 65, 972A-986A (1993). 

There are two definitions for the word novice. The first is that a novice is a person new to or inexperienced in a field. Yes, this book is addressed to novices in mass spectrometry. A novice is also described as a person who has entered a religious order and is under probation before taking vows. Perhaps this is a stretch, but many mass spec-trometrists (particularly the older ones) believe that the field (or art ) of mass spectrometry is not unlike a religious order—once entered, you are in it for keeps. And when is your probation over What do you have to know before you are confirmed When is a novice no longCT a novice Are we all novices forever because there is always something more to comprehend, to learn, and then apply Do even the high priests of the discipline understand the mechanism of electrospray ionization Such meditations are probably way too philosophical. So let s say to the novice mass spectrometrists please just learn what s in this book, and you will be ready to take your vows. 

Konermann, L., Ahadi, E., Rodriguez, AT)., Vahidi, S. (2013) Unraveling the Mechanism of Electrospray Ionization. Anal. Chem. 85 2-9. 

Ahadi, E., Konermann, L. (2012) Modeling the Behavior of Coarse-Grained Polymer Chains in Charged Water Droplets Implications for the Mechanism of Electrospray Ionization. J. Phys. Chem. B 116 104-112. 

Kebarle, P. and Ho, Y. (1997) On the mechanism of electrospray ionization, in Electrospray Ionization Mass Spectrometry... 

Konermann L, Ahadi E, Rodriguez AD, Vahidi S. Unraveling the mechanism of electrospray ionization. Anal Chem. 2013 85 2-9. 

On the Mechanism of Electrospray Ionization Mass Spectrometry (ESIMS)... 

Chapter 1 On the Mechanism of Electrospray Ionization Mass Spectrometry (ESIMS) Table 1.1. Onset Voltages, Von. for Solvents with Different Surface Tension, y... 

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