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EI is the classical ionization method in mass spectrometry.
EI is best suited to relatively non-polar, volatile samples. An EI mass spectrum usually contains the
molecular ion, M+·, and many fragment ions, making EI useful for structural characterization.
Gas phase ions are produced by heating the sample (if necessary) followed by ionization with a 70 eV beam
of electrons.
View the EI Mass Spectrum of 3-nitrobenzyl alcohol
Chemical Ionization
CI is a "soft" ionization technique which, like EI, is restricted to relatively
volatile samples. However,CI is better suited than EI to more polar compounds. A CI mass spectrum
often contains a protonated molecule, [M+H]+, and a few fragment ions. The sample is heated to
generate gas phase molecules which undergo proton transfer reactions with the CI gas. The
choices of reagent gases available in order from harshest (yielding the most fragments) to softest
are: Methane,Isobutane, and Ammonia.
View the CI Mass Spectrum of 3-nitrobenzyl alcohol
Fast Atom Bombardment
FAB is a "soft" ionization method which overcomes many of the limits
of EI and CI. FAB works well for polar,thermally labile compounds. A FAB mass
spectrum often contains a protonated molecule, [M+H]+, and a few fragment ions.
Gas phase ions are produced by bombarding a sample which has been dissolved in
a vacuum compatible solvent such as glycerol or 3-nitrobenzyl alcohol (NBA) with
an energetic (kV range) atom beam.
View the FAB Mass Spectrum of an unknown
Electrospray Ionization
ESI works well for higher molecular weight compounds such as peptides, proteins,
and oligonueotides. An ESI mass spectrum usually consists of a series of multiply-charged
ions, e.g. [M+nH]n+, for a single compound. The sample is dissolved in a suitable
electrospray solvent such as 50:50 methanol:water with 2% acetic acid. This solution
is passed through a metal capillary which is biased at high potential (4-5 kV). The electric
field causes the solution to disperse into small droplets. These small droplets undergo a process
of solvent evaporization and/or Rayleigh disintegration until gas-phase ions are produced.
View the Electrospray Mass Spectrum of Lysozyme
Matrix-Assisted Laser Desorption/Ionization
MALDI, the newest of the ionization methods described here, is the method of choice for
higher molecular weight compounds such as peptides, proteins, and oligonueotides. A MALDI
mass spectrum usually consists of singly charged ions, e.g. [M+H]+, [M+Na]+, [M+K]+ for
each sample component. The sample is mixed with a suitable MALDI matrix such as sinapinic
acid in a ratio of 1:100. This mixture is allowed to dry on a metal surface. A laser beam
is then directed at this surface causing desorption and ionization of sample components.
Atmospheric Pressure Chemical Ionization
As the name implies, APCI is a technique which creates ions at atmospheric pressure.
A sample solution flows through a heated tube where it is volatilized and sprayed into a
corona discharge with the aid of nitrogen nebulization. Ions are produced in the discharge
and extracted into the mass spectrometer. APCI i s best suited to relatively polar,
semi-volatile samples. An APCI mass spectrum usually contains the quasi-molecular ion, [M+H]+.
This technique is used as an LCMS interface because it can accomodate very high (1 ml/min) liquid
flow rates.
View the APCI Mass Spectrum of reserpine
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