How Secondary Electro-Spray Ionization (SESI) works

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Secondary Electro-Spray Ionization (SESI) uses a nano-electrospray that produces a cloud of charging ions. These ions ionize the vapor molecules that are in contact with the cloud.

The exact ionization mechanism depends on the liquid used, the polarity, the analyte, the temperature and the humidity content, but most species follow the schematic reaction below:


[(H2O)nH]+ + M --> [M+H]+

(where N is the size of the water cluster)



SESI operates at high temperature (near the boiling point of the liquid). As a result, n-electrospray droplets are quickly evaporated and converted into charging agents. This provides three advantages:

  • Detection of low volatility species, which normally carry a greater biological significance. Low volatility species require higher temperatures to stay in the gas phase, and specific flow configurations to reduce background levels.

  • Better sensitivity, due to greater reaction rates.

  • Uniform ionization mechanism. the droplets produced by the electrospray are rapidly converted into ions and clusters, reaching the equilibrium. As a result, the majority of the ionization region is dominated by equilibrium ions and clusters therefore modeling and optimizing a uniform SESI ionizer is easier. Knowing that the composition of the ionization region is uniform also facilitates the understanding of the ionization mechanisms.


SESI operates at atmospheric pressure. Compared with low pressure ionization, this offers four main advantages:

  • Great ionization rate: both the charging ions and the molecules are densely packed because of the pressure. This results in faster reactions.

  • Reduced coulombic dilution: Once formed, the pressure of the gas reduces the effect of coulombic repulsion on the ionized sample molecules. The mobility of the ions is lower at higher pressure, thus the ions cannot diverge from the ionization region.

  • Fast response, even for low volatility species: The gas does not need to pass through the complicated interface that communicates the sample and the internal side of the mass spectrometer (MS). Only the ions are passed to the MS. The gas path is designed with only one priority in mind: reduce background levels without compromising to any other criteria.

  • Compatibility: SESI operates upstream of the inlet of the MS. It can be easily interfaced with almost any MS that is compatible with a regular electrospray.

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