Electrospray Ionization Method
Among the most used spray ionization techniques is Electrospray
Ionization (ESI). This technique has been and still being the method of
choice for analyzing thermolabile chemicals. Its capabilities are well
documented. It uses and electrical stress between the ESI probe exit (e.g.
capillary) and the counter electrode, which is located few millimeters
from the probe. The process results in the generation of highly charged
droplets directly from the infused solution. Multiply and/or singly charged
analyte molecules desorbe from the sprayed droplets and sampled through
the rest of the mass spectrometer. ESI has been distinguished for its ability
to produce multiply charged molecular ions from a large variety of polymers
such as protein and DNA fragments; it allows also sensitive detection of
singly charged low molecular weight polar species such as drugs and drug
metabolites. The formation of positive or negative ions (depending on the
sign of the applied electrical field) occurs in high yield. In the positive
ion mode protonated and/or alkali adduct analyte molecules generally observed
in the mass spectra. In the negative ion mode operation peaks corresponding
to deprotonated analyte molecules are observed. ESI is described as a very
"soft" ionization technique where the surrounding bath gas has a moderating
effect on the internal and translational energies of desorbed ions.
Advantages of ESI:
- Soft ionization process so intact molecular ions are
observed
- ESI allows production of multiply charged ions. This
results in the ability of analyzing very high molecular weight species
using the most available mass analyzers (e.g. quadrupoles).
- ESI is an atmospheric pressure process. This makes it
easy to use and easy to interface with HPLC and CE separation techniques.
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