4. Mass Spectrum
The MS of a typical hydrocarbon, n-decane is shown below. The molecular ion
is seen as a small peak at m/z = 142. Notice the series ions detected that
correspond to fragments that differ by 14 mass units, formed by the cleave of
bonds at successive -CH2- units
4
7. How does a Mass
Spectrometer work?
3 fundamental parts: the ionisation source, the
analyser, the detector
Separation in analyser according to mass-to-
charge ratios (m/z)
Detection of separated ions and their relative
abundance
Signals sent to data system and formatted in a
m/z spectrum
7
9. Ionisation methods
Chemical Ionisation (CI)
Electron Impact (EI)
Electrospray Ionisation (ESI)
Fast Atom Bombardment (FAB)
Field Desorption / Field Ionisation (FD/FI)
Matrix Assisted Laser Desorption Ionisation
(MALDI)
Thermospray Ionisation 9
10. FIELD IONIZATION (FI)
• Field ionization technique is useful ionization method ,
appropriate for the analysis of the thermally labile &
high mole weight sample.
• Field ionization a technique as need to develop
ionization technique to complement electron impact,
which is limited to nearly low molecule weight sample
that can be vaporized prior to ionization and often
leads to thermal degradation of involatile or heat
sensitive compounds.
• Field ionization rely on the “mild” or “soft” ionization of
the sample by an intense electric field applied to the
surface on which the sample has been deposited 10
11. Principle
In FI, the sample has bee introduced in to the FI source of
mass spectrometer in it’s vapour phase & this can be achieved
by using a heated probe by eluting heated reservoir inlet such
as AGHIS.
An organic compounds in the gas phase can be ionized when
the molecule pass near a sharp metal anode having a electric
field .
Electrons are sucked from the sample molecules into
incomplete orbitals in the metals & resulting molecular ion are
then repelled towards a slit cathode.
Primary focusing takes place at cathode slit before the ion
pass through the entrance slit of the mass spectrometer to be
focused magnetically & electrostatically. 11
12. PRACTICAL OPERATION OF FI.
Essential requirements/ Necessary equipments.
1. Magnetic sector mass spectrometer under
good vacuum equipped with FI source.
2. An FI probe with a supply of emitter usually
carbon or silicon.
3. A range of volatile solvents to dissolve the
samples.
4. A syringe to load the emitter with sample.
5. A suitable reference material for optimization.
12
13. FIELD IONIZATION SOURCE.
In FI source the ions are formed under high influence
of large electric field.
The FI source works with an insertion probe which
supports field emitters,
It is usually a thin Tungsten wire supported on the
two metals posts that are seated on the an insulated
ceramic base, the emitter wire is covered with
microneedles of pyloric carbon or silicone & is held at
the accelerating potential of the ionization source.
The probe is inserted in to ionization source through a
probe lock. 13
17. Conti…
There are other three ionization
process that takes place in FI source.
1. Cation or anion formation
2. Thermal emission
3. Proton abstraction
17
18. Sample
For sample in FI, which involves the removal of
an electron from molecule a by a quantum
tunneling in presence of strong electrical field.
As a result of voltage applied to the emitter &
the extraction plate, the field strengths are
greatest near the the tips of microneedles
where reddi of curvature are the most
pronounced & this is where field ionization
takes plates. 18
19. Set up for ionization operation
The emitter plays a important role in FI mass
spectroscopy.
FI emitters mounted 0.5 to 2mm from the
cathode, which often serves as slit.
Before setting up for FI, the source temp.
should be set at 180˚c for FI.
Optimization of FI analysis can that be
achieved by monitoring & tuning the ion beam
arising from a known sample or solvent with
oscillatoscope or on the data acquiring system.
19
20. Conti………….
Sample can be introduced through a
reservoir inlet system such as an ALL
GLASS HEATED INLET SYSTEM (AGHIS)
The solvent is useful for tuning purpose
it produces M+ ions at m/z 58 in
positive ionization operation without
any heat being applied to emitter.
20
21. Conti……………
Source accelerating voltage is usually set
according to the mass spectrometer being used
after taking in to account the molecular weight
of samples.
If high mole. Weight of sample, the source
accelerating voltage having low,
Extraction rod potential is then adjusted some
10-15 Kv lower then the accelerating voltage.
21
22. Conti…………..
The gaseous sample from a inlet system is allowed to
diffuse in to high field area around the micro tips of
the anode.
The electric field is then concentrated at the emitter
tips & ionization occurs via quantum mechanical
tunneling mechanism in which electron from the
analyt are extracted by microtips of anode.
Little vibrational or rotational energy is imparted to
the analyte, thus little fragmentation occurs.
22
24. ADVANTAGES
Over the chemical ionization the chief
advantages of FI are,
1. Increased abundance of molecular ions.
2. Minimization of complex fragmentation &
rearrangements.
3. I can be applied to the following classes of
compound that can be difficultly analyzed by
electrons impact ionization.
24
25. Limitation
Some of the limitation are
1. Lower sensitivity
2. Lower resolution obtained.
25
26. APPLICATONS
FI generates predominantly molecular weight information with
little fragmentation on sample of low to moderate polarity.
Biotechnology:
analysis of proteins, peptides, oligonucleotides
Pharmaceutical:
drugs discovery, combinatorial chemistry, pharmokinetics,
drug metabolism
Clinical:
neonatal screening, haemoglobin analysis, drug testing
Environmental:
water, food, air quality (PCBs etc)
Geological:
oil composition 26
27. REFERENCES
Principal of instrumental analysis by SKOOG,HOLLER,NIEMAN,
fifth edition,
Ionization method in Organic Mass spectroscopy by Alison E.
Ashcroft
Organic spectroscopy by William kemp,3rd edition
Spectrometric identification of organic compounds, by
Robert.M.silverstein, francis.X.Webster.6th edition.
www.astbury.leeds.ac.uk/Facil/MStut/mstutorial.htm
(Dr Alison E. Ashcroft at Leeds)
www.asms.org (The American Society for Mass Spectroscopy)
www.spectroscopynow.com
Mass Spec tools
27
