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