FRAGMENTATION TECHNIQUES IN MASS SPECTROSCOPY
Presented by:Sk.Samiya,
1st year M-Pharm,(Pharmacology)Hindu College of Pharmacy
Mass spectroscopyDefinition: It is a powerful analytical technique useful for determination of molecular weight of compound.Principle: Initially in mass spectroscopy the sample must be converted into
vapourised or gaseous form then it undergo ionization to form a molecular ion with the liberation of 1e-.
This molecular ion normally undergoes fragmentation to form several fragmented ions or daughter ions.
FRAGMENTATION The process of breaking up of molecular ion into smaller or daughter
ions is known as “fragmentation”. The molecular ion commonly decomposes to a pair of fragments,
which may be either a radical with an ion or a small molecule & a radical cation.
Fragmentation process
Bombardment of molecules by a electron beam with energy between 10-15ev usually results in the ionization of molecules by removal of one electron (molecular ion formation).
When the energy of electron beam is increased between 50-7ev, these molecular ions acquire a high excitation resulting in their break down into various fragments.
General modes of fragmentation
Fragmentation of the molecular ion takes place in following mods: Simple cleavage1. Homolytic cleavage2. Heterolytic cleavage3. Semi heterolytic cleavage Rearrangement reactions1. Elimination reactions2. Ortho reaction3. Mc-lafferty rearrangement
Simple cleavage: Transfer of electrons takes place
1. Homolytic cleavage In hemolytic cleavage the electrons are equally transferred to both
atoms & it forms radical and cation.
R
H2C
CH
CH2
R
H2C
CH
CH2
R
+CH
HC
CH2
IONISATION HOMOLYTIC CLEAVAGE
2. Heterolytic cleavage
Fragmentation by movement of two electrons: In this type of cleavage both the electrons of the bond are taken over
by one of the atoms; the fragments are an even electron cation and a radical with the positive charge residing on the alkyl group.
C4H9Iionisation
CH3
CH2
+ I
C4H9I
HETEROLYTIC CLEAVAGE
3.Semi heterolytic cleavage
In this one electron bond cleavage takes place resulting in formation of radical & cation.
C
H
H H
H
CH
H HH
CH3+ HIONISATION CLEAVAGE
Rearrangement reaction
It involves the cleavage of bonds and formation of new bonds i.e., it
requires changes to at least two bonds. So the produced ions are not structural units of precursors. Generally rearrangement leads to loss of smaller molecules.
1. Elimination reaction
In his fragmentation hydrogen is obstructed by hydroxyl group or halogen or acetate functional groups results in the elimination of water or neutral molecule.
R – CH2 – CH2 –CH2 R – CH2 – CH2 – CH2 +
OCOCH3 OCOCH3
CH3COOH + R – CH2 – CH = CH2
2. Ortho effect reaction In the case of substituted aromatic compounds the substitute & carbon come into
proximity has help in elimination of neutral molecule. This effect is called ortho effect.
O
O
CH3
CH3
O
O
CH3
CH2
C
O
CH2
+
OCH3
H C
CC
O
CH2
+ CH3OH
methyl 2-methylbenzoate
methyl 2-methylbenzoate ion
(6-methylenecyclohexa-2,4-dienylidene)methanone
H
3. Mc-Lafferty rearrangement reaction
The loss of an alkene fragment by cyclic rearrangement of a carbonyl compounds ( like aldehyde, ketones, acids, esters, amines), unsaturated compounds with γ-hydrogens undergo Mc-Lafferty rearrangement.
Or It involves the migration of γ-hydrogen atom followed by cleavage
of β-bond then rearrangement leads to the elimination of neutral molecule.
Mechanism
Fragmentation takes place only at γ-hydrogen
Then this hydrogen is obstructed by oxygen of carbonyl group or unsaturated compound.
Carbonyl group is converted to hydroxyl group
Bond break down between α & β carbons resulting in liberation of 2e-’s then rearrangement takes place to form a simple alkene.
H
H2CCH2
CH2
CHO
butyraldehyde
H
H2CCH2
CH2
CHO
H2C
CH
CH
CHHO
CH2
CH2
+
OH
C
CH2
H
O
C
CH3
H
H
H2CCH2
CH2
CHO
IONISATION
EXAMPLE: Aldehyde
If m/z value is 44(24+4+16) that indicates aldehyde is present and connected to C3 chain.
H
H2CCH2
CH2
CCH3O
H
H2CCH2
CH2
CCH3O
H2C
CH
CH2
CHCH3HO
CH2
CH2
+
CH2
CCH3HO
CH3
CCH3O
H
H2CCH2
CH2
CCH3
O
pentan-2-one
ionisation
If m/z value is 58 (36+16+6) that indicates the compound containing ketone group
attached to C3 chain
Ex:- ketone
H
H2CCH2
CH2
CNH2O
H
H2CCH2
CH2
CNH2O
H2C
CH
CH2
CHNH2HO
CH2
CH2+
CH2
CNH2HO
CH3
CNH2O
butyramide
H
H2CCH2
CH2
CNH2O
ionisation
Ex:-Amides
If m/z value is 59 (24+16+14+5) that indicates the compound containing amide group
attached to C3chain
REFERENCES:
Instrumental methods of chemical analysis- B.K Sharma,
Instrumental methods of analysis By Chatwal,
Principles of Instrumental analysis By Donglas Skoog