This is related to ionization techniques in MS

1. Ionization Techinques
in Mass Spectrometer

2. In mass spectrometry, Ionization occurs in the ion source
and produce gas phase ion.
A sample is ionized in an ion source and the resulting ions
are separated by their mass to charge ratio. These ions are
detected and the results are displayed as spectra. The relative
abundance of detected ion apeared as a function of mass to
charge ratio.
Molecular ion formed when energy of the electron beam
reached at 10 to 15 ev and fragments are formed at 70ev.
Ion source produce ions without mass discrimination and
accelerates to analyzer.

3.  Ion source is classified into
 Gas phase source
 Desorption sources
Gas Phase Sources
 Electron ionization source
 Chemical ionization source
Desorption sources
Field ionization
MALDI
Electron spray ionization

4. It is known as electron bombardment
ionization.
Main function of this technique is to convert
the gaseous sample into molecular ion .
It is a hard ionization technique because high
energy electron of 70ev used to produce
molecular ion and produce large number of
fragment ion.

5. The 70ev ionizing voltage based on empirical
observation because molecules are most
efficiently ionized in the range of 50 to
90ev.Below 50ev electron do not impart
sufficient energy to target molecule to cause
sufficient ionization and above 90 electron are
so energetic that the did not react with target
molecule.
Typically a mass range 50 to 600 atomic mass
unit is scanned with three seconds.

7. The gaseous neutral sample molecules enter
into the ion chamber through the molecular
leak.
A tight helical beam of highly energetic
accelerated electron beam (70ev) from a
glowing Tungsten or Rhenium filament pass
perpendicular to the incoming gas
molecule.

8.  These electrons are drawn off by a positive charged slit
called as electron trap, which is on the opposite side of the
filament.
 Thus, electrons travel across the ion chamber.
 Ions are generated by the exchange of energy during the
collision of the electron beam and sample molecules.
 As the electron beam is highly energetic (70 eV), it
provides sufficient energy to gas molecules to have
ionization and to cause the characteristic fragmentations
of sample molecules either by loss of radicals or by loss of
neutral molecules.

9. e- (70eV)
M ------------------> M+. + 2е-
M + .------------> A+ + B .
M + . --------------->A+ . + B
The positive ions formed in the ion chamber are drawn out by a
small electrostatic field between the larger repeller plate (positively
charged) and the first accelerating slit (negatively charged).
A strong electrostatic field between the 1st and 2nd accelerating
slits of 400-4000 V, accelerates the ions of various masses to their
final velocities. The ions emerge from the final focusing slit

10. As a collimated ion beam with velocities and Kinetic Energies as
ZV = ½ m1 υ 1
2 = ½ m2 υ 2
2 = ½ m3 υ 3
2 ………….
Where
z = Charge of the ion
V = Voltage
υ = Velocity of various ions
m = Mass of respective ion
Then the collimated ion beam will enter in to the Ion-Analyser,
which sort the ions as per their m/z ratios.

11.  Gives molecular mass
and fragmentation
pattern of the sample.
 Extensive
fragmentation and large
number of peaks give
structural information.
 Good sensitivity.
 Only volatile sample
used.
 Unstable Molecular
fragment ions readily
formed that are absent
from mass spectrum.
 Need thermal stability
DISADVANTAGES

12. INTRODUCTION
• In chemical ionization, the ionization of the
analyte is achieved by interaction of it’s
molecules with ions of a reagent gas in the
chamber or source.

13. • Chemical ionization is carried out in an
instrument similar to electron impact ion source
with some modifications such as:-
 Addition of a vacuum pump.
Narrowing of exit slit to mass analyzer to
maintain reagent gas pressure of about 1 torr
in the ionization chamber.
 Providing a gas inlet.

15. It has two part processes.
First Step:
A reagent gas is ionized by Electron
Impact ionization in the source.
The primary ions of reagent gas react with
additional gas to produce stabilized reagent ions.

16. Second Step:
• The reagent ions interact with sample
molecules to form molecular ions.
• The sample is diluted with a large excess of
reagent gas to reduce chances of ionizing
collisions between sample molecules and the
electrons.
• Primary ions are formed entirely from
interaction with reagent gas ions.

17. • Gases commonly used as reagent are low
molecular weight compounds such as Methane,
tertiary Isobutane, Ammonia, Nitrous oxide,
oxygen and hydrogen etc.

18. Depending upon the type of ions formed CI is
categorized as:-
1. Positive Chemical Ionization
2. Negative Chemical Ionization

19.  In this technique positive ions of the sample are
produced. In positive chemical ionization gases such as
Methane, Ammonia, Isobutene etc are used. For
example Ammonia is used as reagent gas.
 First ammonia radical cations from generated by
electron impact and this react with neutral ammonia to
from ammonium cation (reactive species of ammonia
Cl).
NH3
e- NH3
+ +2e-
NH3
+ NH4
+ +NH2
 NH4
+ reacts with the sample molecules by proton
transfer or Adduct formation to produce sample ions.

20.  M + NH4
+ [M + H] + + NH3 Proton transfer
 M +NH4
+ [M + NH4] + Adduct formation
 When Methane is used as Reagent gas. Methane is ionized by
electron impact:
CH4
+ + e- CH4
+ + 2e-
 Primary ions react with additional reagent gas molecules to produce
stabilized reagent ions.
CH4
+ + CH4 CH5
++ CH3
CH3
+ + CH4 C2H5
+ +H2
 The reagent ions then react with the sample molecules to ionize the
sample molecules:
CH5
+ +MH CH4 +MH2
+ (proton transfer)
CH3
+ +MH CH4 + M+(hydride abstraction)
CH4
+ +MH CH4 + MH+(Charge transfer)

21.  Negative chemical ionization is counterpart of
Positive chemical ionization.
 In this technique, negative ions of the sample
are formed.
 Oxygen and Hydrogen are used as reagent
gasses.
This method is used for ionization of highly
electronegative samples.

22. The negative ions are formed by following
reactions :-
Resonance electron capture
M + e- M-
Dissociative electron capture
RCl + e- R + Cl-
H2O + e- H + OH-

23. The ion molecule reaction occurring between
negative ion formed in the chamber source
and the sample molecule include:-
Charge transfer.
Hydride transfer.
 Anion- Molecule adduct formation.

24. Used for high molecular weight compounds.
Used for samples which undergo rapid
fragmentation in EI.
Require little to no additional equipment over
electron ionization.
Require lower energy than EI.
Milder ionization which reduce fragmentation.

25. Not suitable for thermally unstable and non-
volatile samples.
Relative less sensitive then EI ionization.
Samples must be diluted with large excess of
reagent gas to prevent primary interaction
between the electrons and sample molecules.

26. Introduction:
Field ionization is a technique which is used to
produce ions from volatile compounds that do
not give molecular ions by EI.
It produces molecular ions with little or no
fragmentation.
Applications of very strong electric field
induces emission of electrons.

28.  In this technique, sample molecules in vapor
phase is brought between two closely packed
electrodes in the presence of high electric field
(107-108V). It experiences electrostatic force.
 If the metal surface (anode) has proper geometry
(a sharp tip, cluster of tips, or a thin wire) and is
under vacuum (10-6 torr) this force is sufficient to
remove electrons from the sample molecules
without imparting mush excess energy.

29. The electric field is produced by applying high
voltage (20kV)to these specially formed
emitters (made up of thin tungsten wire).
In order to achieve high potential gradient
necessary to effect ionization, the anode is
activated by growing carbon micro needles or
whiskers.
These whiskers are 10 micro meters in length
and greater then 1um in diameter.

30. Valence electron are removed from organic
molecules by mechanical tunneling mechanism.

31. Protonated specie is formed as the result of
ion-molecule reaction.
Thus M+ is observed in field ionization
spectrum.
These cations are accelerated out of source
and their mass is analyzed by analyzer.

32. As fragmentation is less, abundance of
molecular ion is enhanced.
This method is useful for relative molecular
mass and empirical formula determination.

33. Not suitable for thermally unstable and non
volatile samples.
 No structural information is produced as very
little fragmentation occurs.

34.  Michael D. Sefcik, Jay M. S. Henis, Peter P. Gaspar. The methanium ion,
CH5+. Evidence for the structure of a nonclassical ion from reaction
studies by ion cyclotron resonance spectroscopy. The Journal of Chemical
Physics 1974, 61 (10) , 4321-4328. DOI: 10.1063/1.1681738.
 Allan B. Foster, Michael Jarman, John D. Stevens, Peter Thomas, John H.
Westwood. Isotope effects in O- and N-dimethylations mediated by rat
liver microscopes: An application of direct insertion electron impact mass
spectrometry. Chemical-Biological Interactions 1974, 9 (5) , 327-340.
DOI: 10.1016/0009-2797(74)90128-8.
 Alfred L. Yeorgi, Frederick W. Lampe. Carbon gasification in the
Boudouard reaction. Fuel 1974, 53 (4) , 280-281. DOI: 10.1016/0016-
2361(74)90049-0.
 M. A. Winnik. Intramolecular catalysis in the mass spectrometer:
Mechanisms for loss of methanol from methyl esters upon electron-
impact. Organic Mass Spectrometry 1974, 9 (9) , 920-951. DOI:
10.1002/oms.1210090912.