field desorption, field ionisation

7/27/2019 field desorption, field ionisation

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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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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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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.

field desorption, field ionisation

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