By Adem Lewis / in , , , , , , , , /

[MUSIC PLAYING] Hi, my name is Helen. I work at Waters Corporation
as an analytical scientist. Today I’m going to talk about
electrospray ionization. Electrospray ionization,
commonly referred to as ESI, is an example of an atmospheric
pressure ionization technique. Other examples include
atmospheric pressure chemical ionization (APCI), and
atmospheric pressure photo ionization (APPI). ESI for LC-MS in
its current form was formally proposed by
John Fenn and his coworker, Masamichi Yamashita
in the early 1980s. But processes very
similar to ESI were observed and reported
as far back as 1914 and even earlier. In an ESI ion source, a
narrow metal capillary is held at high voltage
and a potential difference is produced between the
capillary and the orifice of the mass spectrometer. A solution containing analyte
ions, often an LC eluant, is sprayed from the capillary. Usually a flow of
gas is also employed to more efficiently nebulize,
or break up, the liquid stream into tiny droplets. ESI requires the formation
of ions in solution prior to the transfer
of the gas phase. Typically, molecules
are believed to undergo
electrochemical reactions, either from redox reactions
at the liquid metal interface of the capillary
tip or through acid base reactions in solution. The exact mechanism by which
ions are transferred in the gas phase is still a matter for
debate and ongoing research. But two main processes
have been proposed. 1) The ion evaporation
mechanism (IEM), where the electric field of
the surface of highly charged small droplets becomes
sufficient to field desorbed ions directly
from the surface. In this process,
solvated ions leave the larger droplet
sequentially and the droplet shrinks until just
dissolved ions remain. 2) For the charge
residue module (CRM), where ions eventually become
de-solvated as solvent molecules leave the
droplet surface. In this process,
solvent molecules evaporate from
the larger droplet and the droplet shrinks until
just de-solvated ions remain. Evidence suggests
that smaller ions are more likely to enter
the gas phase by the IEM, whereas larger multiple
charged species are more likely to follow the CRM. Modifications or
related processes to these two mechanisms
have also been proposed. In contrast to vacuum
ionization techniques, such as Electron Ionization,
or EI as it’s usually known, ESI is considered to be a
soft ionization technique. This is because ESI typically
produces intact ions related to the analyte molecule
and fewer fragment ions than vacuum
ionization techniques. In EI to form positive
ions, high energy electrons impact with
the analyte molecules, knocking an electron from the
outer shell of the molecule and typically
imparting enough energy to break up the molecular
ion to produce fragment ions. In ESI, to form positive
ions, cations for example, hydrogen ions often just
called protons interact with the analyte
molecules and form new bonds between the cations
and the analyte molecules. This produces adduct ions,
also sometimes called cationized molecules,
with a total mass equal to the mass of
the analyte molecule plus the mass of the cation. It is also possible to
form negative ions in ESI by forming adduct ions of
anions, such as chloride ions. Negative ions in
the ESI can also be formed by other processes,
such as proton abstraction, in which a proton is removed
from an analyte molecule to leave a negatively
charged ion. You can learn more about these
processes in the video covering adduct formation. For background reading and
further information about ESI, you could take a look
at these references. [MUSIC PLAYING]

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