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IONISATION TECHNIQUESIN MASS
SPECTROSCOPYField Ionisation (FI) and Field Desorption (FD)
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FIELD IONISATION (FI)AND FIELD DESORPTION (FD)
H. D. Becky in 1969 introduced the concept of FD-
MS for analysis of large organic molecules.
FI and FD are "soft" ionisation methods; i.e. theincrease in the internal energy transferred to the
analyte molecules during ionisation is minimal and
the sunsequent fragmentation is largely reduced.
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FIELDIONIZATION (FI)
Field ionization (FI) is the generation of M+ ionsby removal of electrons, primarily from gas samplemolecules, using a high electric field.
This generally occurs at a sharp edge or tip that isbiased to a high electrical potential
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During Field Ionization, sample molecules become ionized
by the quantum tunneling of a valence electron as they
pass close to the tips of emitter electrodes.
The electrodes are essentially a large collection of carbon
micro-needles, deposited on tungsten wire, surrounded by a
very high electric field within the ion source.
These micro-needles allow very high electric field strengths
to be applied to a molecule.
The mechanism of ionisation is based on the fact that when
a molecule is subjected to a very high electric field, a
valence electron tunnels through the potential barrier and is
removed from the molecule. The resulting ion is therefore
M+.
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FIELD IONIZATION
Field Ionization offers the following benefits:
Suitable for substances that can be introduced into
the source via a gas chromatograph or heated
direct insertion probe
Energetically gentle ionization process providing a
large proportion of molecular ions from many
different substances
Spectra exhibit some fragmentation due to the heat
used for sample volatilization
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FIELDDESORPTION (FD)
Field desorption (FD) is a method for emitting ionsinto the gas phase.
Sample spread on an emitter is heated while a highelectric field is applied.
Ions are then emitted by the tunneling, ion-moleculereactions, thermal fusion effects,
and other phenomenon
occurring on the emitter
surface and around the
whisker ends. The ionization phase depends strongly on the
sample material and the spread condition.
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FIELD DESORPTION
Field Desorption analysis feature the following:
Suitable for high molecular mass and/or thermally labilesubstances such as polymers, peptides, carbohydratesand organic or inorganic salts
A solution of the sample is applied to the emitter before
it is introduced into the ion source The emitter is mounted on the tip of the axial sample
introduction probe
Samples undergo little or no fragmentation during FDionization
Ionization maximizes the production of high-qualitymolecular mass information for improved compoundidentification/confirmation
FD-MS has proven capabilities in analyzing nonioniclow- to medium polarity compounds
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FIELD IONIZATION (FI) VERSUS FIELD
DESORPTION (FD)
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FIELD IONIZATION (FI) VERSUS FIELD
DESORPTION (FD)
FI : sample is heated
in a vacuum so as to
volatilize it onto an
ionization surface. FI issuited for use with
volatile, thermally
stable compounds.
FI sources are
arranged to function
also as FD sources
FD : the sample is
placed directly onto
the surface (dipping
emitter in an analyte
solution) before
ionization but FD is
needed for nonvolatile
and/or
thermally labilesubstances.
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FIELD IONIZATION (FI) VERSUS FIELD
DESORPTION (FD)
FI Spectrum for D-Glucose FD Spectrum for D-Glucose
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LIQUID INTRODUCTION FIELD DESORPTION
IONISATION
(LIFDI)
A major advance was made by H B Linden in 2000 byincorporating a fused silica capillary to allow liquidintroduction of the analyte onto the FD emitter, hence theacronym LIFDI.
LIFDI is regarded as a convenient tool to ionize any solid,liquid, or gaseous species transferred to the emitter insideof the ion source through a capillary without breakingvacuum.
This development made the FD technique much more
user friendly and, because the emitter remains in place,the fragile emitters last much longer.
The fused silica capillary also enables air and moisturesensitive molecules to be easily introduced without
exposure to ambient air.
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The entire LIFDI sample preparation is done by
dipping the capillary into the sample solution for 1-2
s. A volume of 40 nL is aspirated automatically and
forced through the capillary.
Upon arrival at the emitter, osmotic and capillary
forces between the dendrites distribute the solution
over the entire emitter.
The small volume of solvent is evaporated in the
vacuum within seconds, followed by acquiringspectra at a total sample prep of less than 30 s.
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APPLICATIONS
Identification of Organometallic compounds using FieldDesorption Ionization Technique.
Qualitative and quantitative analysis for drugs and
endogenous compounds, e.g., tranquillizers,
immunosuppressive and antitumour agents and free
amino acids, in human body fluids, biocides, e.g.,
phenylureas, carbamates, organophosphorus and
organometallic compounds, in river water, and natural and
synthetic products, e.g., saponins, chlorophyll and
deoxyribonucleotides, can be performed Field desorption mass spectrometric profile analysis can
be used as a technique for the detection of
biotransformation products of xenobiotics in crude urine
extracts.