In the equilibrium, both Secondary Electro Spray Ionization (SESI) and Atmospheric Pressure Chemical Ionization (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 fragmentation 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.
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.