Page 261 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Mass Spectrometry 149

The movement of sample through the CE column is ization process is the direct result of the interaction of an
driven by a potential difference between the entrance and energetic electron with the electrons in the molecule of
the exit of the CE column; this applied potential must interest. The electrons are emitted from a metal (usually
be taken into account when designing an interface that rhenium) filament through which 3–4 Amperes of current
◦
directs the effluent into the ionization source of the mass is passed. This current heats the filament to about 2000 C;
spectrometer. CE/MS is exclusively coupled with the ESI electrons are released from the metal and accelerated into
source, which has its own requirements for imposition of a the source. The classic “70 eV” electron ionization mass
high potential on the needlefromwhichthesprayemerges. spectrum of an organic compound is obtained when the
Several early designs were explored for integration of both potential difference maintained between the filament and
solvent flow and imposed potentials. Integrated designs thesourceblockis70V,withtheblockbeingmaintainedat
now use the exit of the CE column itself as the spraying a more positive potential. Variations in the measured mass
tip for electrospray. A relevant point in all designs is the spectra and in the ionization cross sections of organic com-
fact that the CE column exits at atmospheric pressure, pounds with changes in the electron energy were studied
which preserves the pressure profile within the column, early in the development of electron ionization mass spec-
and maintains the inherently high CE resolution. trometry. A value of 70 V was chosen so that mass spectra
recorded at this electron energy did not vary greatly with
small changes in the electron energy, and the sensitivity
B. Ionization Sources
(number of ions produced per amount of sample intro-
Mass analyzers cannot manipulate neutral molecules. A duced into the course) is also essentially constant about
positive or negative charge is necessary for interaction of this value.
the ion with magnetic and electric fields. The differing The incident electrons must have an energy greater than
magnitudes of responses for ions of different masses is the ionization energy of the target gas molecule M, defined
the basis for their separation in the mass analyzer. Most as the energy required to remove the electron held most
samples exist initially as neutral molecules, and gas chro- weakly within the molecule. The electron ionization pro-
matography and liquid chromatography are used for the cess can be written for the gas-phase sample molecule M:
most part to separate mixtures of components that are
M (gas) + e − −→ M + + e − + e − .
neutral in the gas phase, and neutral in the liquid used filament filament molecule
as the LC solvent. The ionization source converts these Electrons in molecular orbitals are moving at velocities
8
neutral molecules into ions, or extracts ions from solu- of about 10 m/sec. As the very fast filament electron ap-
tion, and passes the ions into the mass analyzer of the proaches the molecule, it causes the release of one of the
instrument. The physical means to do so depends on the slower molecular electrons, forming a positively charged
initial form of the sample. Electron and chemical ioniza- molecular ion. The molecular ion M + (the superscripted
tion are the older and more traditional ionization meth- dot denotes an unpaired odd electron) may subsequently
ods, both widely used with GC. Liquid chromatography dissociate, since the ionization imparts more energy to the
is used predominantly with electrospray ionization, as is molecule than that required for ionization alone. The ex-
capillary electrophoresis. MALDI is an ionization method cess energy can cause the dissociation of the molecular
that creates ions directly from a solid mixture deposited on ion M , or it can be retained in the ion as excess internal
+
a surface (consistent with planar chromatography, but not energy. Since an electron is far too light to transfer kinetic
directly with the column-based chromatographic meth- energy to the sample molecule in a collisional process, the
ods that dominate modern mass spectrometry). MALDI process of electron ionization involves electronic excita-
is included here since it is one of the rapidly growing ap- tion of M. The assumption that the atoms do not move as
plications areas in biological mass spectrometry, and its the transition to an excited electronic energy state occurs
use may revitalize some forms of planar chromatography, is known as the Franck–Condon principle. The assump-
including forms of planar gel electrophoresis and affinity tion states, in effect, that the molecular ion M + retains the
chromatography. original structure of the molecule M, at least at short times
after its formation. If dissociations of the molecular ion
areprompt,therefore,wecanassumethatthedissociations
1. Electron Ionization
represent those of the original molecule and not a struc-
Electron ionization was the first ionization method de- turally reorganized isomer. Some fraction of the molecular
veloped for mass spectrometry, and it remains the most ions formed will be stable enough to pass into the mass
widely used. The most extensive mass spectral libraries analyzer; their measured m/z ratio is a direct indication of
assembled are those of electron ionization mass spectra the molecular mass of the sample molecule itself. Ratio-
recorded under a “standard” set of conditions. The ion- nalization of the processes that lead to the fragmentation