Principle of Operation
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. SUPER SESI is optimized for the 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 design 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.
Diferences between SESI and APCI:
In the equilibrium, both SESI and APCI generate charged water clusters that act as charging agents. They differ in how these charging agents are produced.
In SESI, charging agents are produced from evaporating nano-droplets. No high energies involved.
In APCI, charging water clusters are the final result of a much more complex cascade of charge transfer reactions that starts with much greater energies.
This high energy region makes the difference between SESI and APCI. The higher energies involved in APCI produce some level of fragmentation, whereas fragmentation in SESI is much rarer.
Plasma region produces fragmantation and complex spectra. When a complex sample is analyzed, or if an analyte of interest is very diluted and accompanied by many species at higher concentrations, the little fragments produced by the APCI plasma region and the more concentrated species, rise the background levels, thus rendering detection of diluted species impossible.
Cleaner spectra is key to detect diluted species in complex samples. With no plasma fragmentation, each species in SESI produces only the main peak*, in contrast with APCI, which produces much more complex spectra than SESI. This is especially the case if you are analyzing complex samples, in which the analyte of interest is very diluted and accompanied by many species at higher concentration.
The charge transfer reactions are specific, very efficient, and very soft (no high energies involved). As a result, SESI enables:
A very high ionization efficiency.
Soft ionization of polar species with no fragmentation.
An instantaneous response.
Low volatility species
High volatile species can be detected, even when they are present in very low concentrations, with current VOC MS analyzers. For instance, in 2009, P. Spanel and D. Smith measured the concentration of ammonia, acetone, methanol, ethanol, and isoprene in human breath using Selected Ion Flow Tube Mass Spectrometry (SIFT-MS)  .In another study by B. J. Prince, M.J. McEwan et. al , 1,3-Butadiene was detected at a concentration of 9 ppt  in ambient air. This study also measured Toluene, Benzene, and Ethanol. Proton Transfer Reaction Mass Spectrometry (PTR-MS) has been used in several applications to detect VOCs at very low concentrations, providing limits of detection in the ppt range.
things get more complicated with low volatility species
With a ppt sensitivity, and if sensitivity was the only limiting factor, one would expect to be able to detect very low volatility species, with vapor pressures in the range of 10 -12 Bar. However, the less volatile species mentioned in the recent review on PTR-MS are Phenol and Aniline  (See Table 1 of the mentioned review). The vapor pressures of Aniline and Phenol are 5·10 -4 Bar and 8·10 -4 Bar, which are much higher than the theoretically expected limit of 10 -12 Bar for a sensitivity in the ppt level. This mismatch of over seven orders of magnitude shows that VOC analysis technology has a gret potential for improvement.
super sesi is optimized for low volatility species.
Over the last years, we dramatically improved the ionization efficiency, but other important effects hold the Limits of Detection (LoD). SUPER SESI has greatly improved background levels, which are dominated by condensation effects. Currently, these are the limiting factors defining the LoD for low volatility species.
-  Patrik Spanel and David Smith; Progress in SIFT-MS: breath analysis and other applications; Mass Spectrometry Reviews, 2011, 30, 236– 267;
-  B. J. Prince, D. B. Milligan and M. J. McEwan; Application of selected ion flow tube mass spectrometry to real-time atmospheric monitoring; Rapid Commun. Mass Spectrom. 2010; 24: 1763–1769
-  X. Zhan, J. Duan, and Y. Duan; Recent developments of proton-transfer reaction mass spectrometry (PTR-MS) and its applications in medical research; Mass Spectrometry Reviews, 2013, 32, 143–165