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Fragmentation techniques in mass spectroscopy

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Fragmentation techniques in mass spectroscopy

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Fragmentation techniques in mass spectroscopy

  1. 1. FRAGMENTATION TECHNIQUES IN MASS SPECTROSCOPY Prepared by MAHENDRA.G.S 1 M pharm Department of Pharmaceutical chemistry J S S College of Pharmacy Mysuru
  2. 2. Definition: 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.
  3. 3.  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.
  4. 4.  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.
  5. 5. Fragmentation of the molecular ion takes place in following mods:  Simple cleavage 1. Homolytic cleavage 2. Heterolytic cleavage 3. Semi heterolytic cleavage  Rearrangement reactions 1. Elimination reactions 2. Ortho reaction 3. Mc-lafferty rearrangement
  6. 6.  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
  7. 7.  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.C4H9I ionisation CH3 CH2 + I C4H9I HETEROLYTIC CLEAVAGE
  8. 8.  In this one electron bond cleavage takes place resulting in formation of radical & cation. C H H H H C H H H H CH3 + H IONISATION CLEAVAGE
  9. 9.  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.
  10. 10.  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
  11. 11.  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 C H2 C O CH2 + OCH3 H C C C O CH2 + CH3OH methyl 2-methylbenzoate methyl 2-methylbenzoate ion (6-methylenecyclohexa-2,4- dienylidene)methanone H
  12. 12.  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.
  13. 13. 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.
  14. 14. H H2C C H2 CH2 C HO butyraldehyde H H2C C H2 CH2 C HO H2C C H CH C HHO CH2 CH2 + OH C CH2 H O C CH3 H H H2C C H2 CH2 C HO IONISATION EXAMPLE: Aldehyde If m/z value is 44(24+4+16) that indicates aldehyde is present and connected to C3 chain.
  15. 15. H H2C C H2 CH2 C CH3O H H2C C H2 CH2 C CH3O H2C C H CH2 CH CH3HO CH2 CH2 + CH2 C CH3HO CH3 C CH3O H H2C C H2 CH2 C CH3 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
  16. 16. H H2C C H2 CH2 C NH2O H H2C C H2 CH2 C NH2O H2C C H CH2 CH NH2HO CH2 CH2 + CH2 C NH2HO CH3 C NH2O butyramide H H2C C H2 CH2 C NH2O ionisation Ex:-Amides If m/z value is 59 (24+16+14+5) that indicates the compound containing amide group attached to C3chain
  17. 17. REFERENCES:  Instrumental methods of chemical analysis- B.K Sharma,  Instrumental methods of analysis By Chatwal,  Principles of Instrumental analysis By Donglas Skoog

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