Things you didn't know about SESI and ESI (Electro-Spray ionization)

Things you didn't know about SESI and ESI (Electro-Spray ionization)

INITIAL HISTORY

SESI and ESI (Electro-Spray Ionization) have been linked since the initial development of ESI. Fenn and colleagues already noted that gas-phase species in the counterflow gas were also ionized by the spray. Still today, the routine calibration of mass spectral data relies on ubiquitous plasticizes and other contaminants (i.e. polydimethylcyclosiloxanes, phthalates) that are ionized in regular ESI via Secondary Electrospray Ionization mechanisms. Indeed, most laboratories and mass spectrometers (MS) users have databases of identified contaminants, which appear in the mass spectra regardless of the liquid sample. The persistence of the signals produced by these commonly known contaminants shows that the ionization mechanism of SESI is very effective.

In an early stage, several authors started to demonstrate the potential of SESI-MS in different applications using home-made non-optimized SESI sources. H. Hill et. al. coined the term SESI, and used it to demonstrate its suitability for detecting vapor traces of illicit drugs [1]. P. Sinues showed that it could be used to detect volatiles of explosives [2], volatiles released by the human skin [3], and to analyze the content of trace metabolites in breath [4], [5], and J. Hill was able to characterize and differentiate bacterial cultures with SESI-MS [6]. 

DROPLET VS ION PATHWAY

Initial discussions on the ionization mechanism considered two possible ionization mechanisms:

  • Droplet pathway: gas phase molecules are first adsorbed by the electrospray droplets, and then as the droplet collapses, they are ionized via regular ESI ionization mechanisms.

  • Ion pathway: the ions or ion clusters produced by the electrospray transfer their charge to the gas phase molecules when they collide.

This discussion is still open when large droplets and ions coexist. However, SUPER SESI uses a nano-electrospray very close to the boiling point of the liquid. This results in very small nano-droplets that evaporate very quickly. The region containing droplets is extraordinarily small, and the ionization region is mostly populated by ions (and clusters) in the lowest energy level.

Having one single ionization mechanism is important for two reasons:

  • This eases studying and understanding the ionization mechanism

  • This simplification allows us to develop specific numerical models, which are at the heart of the optimization

We have worked in SESI-MS for 10 years now, if you would like to know more about how our core solution SUPER SESI works, contact us to ask any questions or keep on reading here.