Mass Spectroscopy

Types of ions, fragmentation modes, fragment characterstics, applications

1. Pr es e n t e d B y : - RAHUL KRISHNAN.P.R M.Pharm 1st Year Dept.Of Pharmaceutics Mass Spectrometry 1 1

2. Contents Introduction of Mass spectrum. Types of Ions Fragmentation modes Fragment characteristics Applications References 2 Grace College Of Pharmacy

3. Introduction of MS  The impact of a stream of high energy electrons causes the molecule to lose an electron forming a radical cation .  A species with a positive charge and one unpaired electron + e - H H C H H H H C H H + 2 e - 3 Molecular ion (M + ) m/z = 16 Grace College Of Pharmacy

4. Introduction of MS  Only cations are detected. - Radicals are “invisible” in MS  The amount of deflection observed depends on the mass to charge ratio (m/z). -Most cations formed have a charge of +1 so the amount of deflection observed is usually dependent on the mass of the ion. . 4 Grace College Of Pharmacy

5. Mass Spectrum The resulting mass spectrum is a graph of the mass of each cation vs. its relative abundance. Relative abundance of an ion means the % of total ion current. Mass spectrum is an analytical techniques which can provide information concerning the molecular structure of organic comp. Base peak is the highest peak or the most intense peak in the spectrum. 5 Grace College Of Pharmacy

6. Types of Ion  Types of ion produced in MS Molecular ions (parent ion) Metastable ions Fragment ions (Dissociation process) 4.Rearrangement ions 5.Multiple charged ions 6.Isotopes ions 7.Negative ions 8.Base peak 6 Grace College Of Pharmacy

7. Molecular ion  Molecular ion (parent ion): -The radical cation corresponding to the mass of the original molecule  The molecular ion is usually the highest mass in the spectrum H H C H H H H H C C H H H 7 Grace College Of Pharmacy

8. Molecular ion When a sample sub.is bombarded with electrons of energies of 9 to 15eV, the molecular ions produced by loss of a single electron. This will give rise to a very simple mass spectrum with essentially all of the ion appearing in one peak called parent peak. M + e = M + + 2e - Most important ion. 8 Grace College Of Pharmacy

9. Molecular ion The relative height of parent peak decreases in the following order aromatic>conjugated olefins>sulphides> unbranched>hydrocarbon>ketones>amine>ester> ethers >carboxylic acid>branched hydrocarbons. 9 If a molecule yields the parent peak due to molecular ion ,the exact molecular weight can be calculated. The peak intensity of the molecular ion differs from one compound to other. Grace College Of Pharmacy

10. The Molecular ion Peak 10 Grace College Of Pharmacy

11. Isotope Ion Many elements have the natural isotopes. For example, Chlorine with mass number 37 exists in addition to Chlorine 35. The presence of isotopes readily produces the isotope ions in the spectrum accompanied by a main molecular ion peak and fragment peaks. 11 Grace College Of Pharmacy

12. Isotope Ion 12 Grace College Of Pharmacy

13. Fragment ion The molecular ion produced in MS is generally left with considerable excess energy. This energy is rapidly lost by the molecular ion resulting in one or more cleavages in it with or without some rearrangement. One of the fragment retains the charge where as the remaining fragment may be stable molecule or radicals. 13 Grace College Of Pharmacy

14. Fragment ion The energy required to remove one electron from the neutral parent molecule is usually 10eV. But the energy of the bombarding electron will be around 70eV. The additional energy is consumed in fragmenting the parent ion. E.g. : Ethyl chloride. CH 3 -CH 2 -Cl + e- = CH 3 -CH 2 -Cl + + 2e- CH 3 -CH 2 -Cl + = CH 3 -CH 2 + + Cl. Or CH 2 -CH 2 + + HCl. (Fragment ion) 14 Grace College Of Pharmacy

15. Fragment Ion peak 15 Grace College Of Pharmacy

16. Metastable ion  The ions in a mass spectrometer that have sufficient energy to fragment sometime after leaving the ion source but before arriving at the detector.  M + A + + N   M + with large amount of internal energy will fragment in the ionization source, producing “normal” A + ions. These A + ions will be seen as narrow peaks at m/z values correct for the mass and charge on the ion A + . M + having only a small excess of internal energy, reach detector before decomposition can occur. Narrow peaks for “normal” M + appear 16 Grace College Of Pharmacy

17. Metastable ion M + which posses excesses of internal energy that are in between the those in above two cases, may fragment after leaving the ion source and before reaching the detector. The product ions, A+, are seen in the mass spectrum as broad peaks, centered at m/z values that are nor correct for the mass and charge on the ion A + . These broad peaks are called “metastable ion peaks ”. The metastable ions are detected differently form normal ions is due to their different momentum. 17 Grace College Of Pharmacy

18. Metastable ion Peak 18 Grace College Of Pharmacy

19. Rearrangement Ion These are commonly formed due to the rearrangement of the fragmented ions. In most of the cases, rearrangement takes place with respect to hydrogen, by a reaction known as McLafferty Rearrangement . It involves the migration of a ϒ hydrogen atom, followed by cleavage of a β bond. 19 Grace College Of Pharmacy

20. Multi-charged Ions These ions may exist in 2 or 3 charges instead of a usual single charge. These are known as doubly or triply charged ions and peaks due to these ions are known as multicharged ion peak. M + e - → M ++ + 3e - M + e - → M +++ + 4e- common in case of the heteroaromatic compounds Fixed gases such as co 2 , N, O etc have measurable peaks corresponding to co 2 +2, N+2, O+2 20 Grace College Of Pharmacy

21. Negative ions . In addition to the positive ions, negative ions are also formed during the electron bombardment of the sample. This results due to the capture of the electrons during the bombardment. These are not observable with the usual mass spectrophotometer, unless some modifications are made, and hence are generally ignored during studies. 21 Grace College Of Pharmacy

22. Base Peak . The largest peak in the mass spectrum corresponding to the most abundant ion or the most intense peak in the spectrum is known as the base peak. Depending on the nature of the compound, it may be a fragmented ion peak or parent ion peak. Sometimes the molecular ion peak may be the base peak, and in that case, it is easy to find out the molecular weight of the compound. The base peak will be assigned with a value of 100% All other peaks are reported as percentages of base peak. 22 Grace College Of Pharmacy

23. Base Peak . 23 Grace College Of Pharmacy

24. FRAGMENTATION MODES The R elative abundance of fragment ion formed depends upon 1) The stability of the ion 2) Also the stability of radical lost. The radical site is reactive and can form a new bond. The formation of a new bond is a powerful driving force for ion decompositions The energy released during bond formation is available for the cleavage of some bonds in the ion. . 24 Grace College Of Pharmacy

25. FRAGMENTATION MODES 1.Simple Clevage i ) Homolytic Clevage a)Mode1 b)Mode2 c)Mode3 ii) Heterolytic clevage 2.Retro-Diel’s-Alder reaction 3.Hydrogen transfer rearrangements 4.Mc Lafferty Rearrangements 25 Grace College Of Pharmacy

26. Fragmentation modes  1) Homolytic cleavage : odd electron ions have unpaired electron which is capable of new bond formation. Bond is formed , energy is released , help offset the energy required for the cleavage of some other bond in the ion. Homolytic cleavage reactions are very common. It is divided into 3: Mode 1 Mode 2 Mode 3 26 Grace College Of Pharmacy

27. Mode1 Operates in compounds where hetero atom is singly bonded to a carbon atom. Parent ion is formed by removal of one electron from hetero atom. New bond is formed with adjacent atom through donation of the unpaired electron and transfer of electron from adjacent bond. 27 Grace College Of Pharmacy

28. Mode 2 When hetero atom is attached to the carbon atom by double bond, α clevage is the preferred fragmentation mode. It is shown by ketones , aldehydes , esters, amides etc. 28 Grace College Of Pharmacy

29. Mode 3 Mainly occurs in the aromatic compounds. It involves the cleavage of c-c bond, which is β to the aromatic ring. 29 Grace College Of Pharmacy

30. Heterolytic Clevage It may be noted the cleavage of C-X (X= 0,N,S,Cl) bond is more difficult than that of C-C bond. In such cleavage , the positive charge is carried by the carbon atom and not by the heteroatom. A s size of the halogen atom attached to the carbon atom increases, the c-x bond becomes very weak, and will undergo easy fragmentation. R-CH 2 -Cl .+ = Cl. + RC + H 2 30 Grace College Of Pharmacy

31. Fragmentation modes 2) Retro –Diels –Alder reaction The reaction is an example of multicentre f r a g m e n t a t i o n w h i ch i s c h a ra cte ri st i c o f cyc l i c o l e f i n s. It involves the cleavage of two bonds of a cyclic system which result the formation of 2 stable unsaturated fragment in which 2 new bonds are formed. This process is not accompanied by any hydrogen transfer rearrangement. The charge can be carried by any one of the fragment. 31 Grace College Of Pharmacy

32. Fragmentation modes 2) Hydrogen ion Transfer Rearrangements Transfer of the Hydrogen atom from one part of the molecule to another. Generally the fragmentation occurs through a six membered transition states. + C 6 H 5 -NH-C=O C 6 H 5 NH 2 + + H 2 C=O I H-CH 2 32 Grace College Of Pharmacy

33. Fragmentation modes 3 ) Mc Lafferty Rearrangement It involves the migration of γ hydrogen atom followed by the cleavage of β bond. To undergo a Mc Lafferty Rearrangement a molecule must possess An appropriately located heteroatom e.g. O, N A pi electron system ( usually a double bond) & An abstractable hydrogen atom gamma to the C = X system Rearrangement leads to the elimination of neutral molecules from aldehydes , ketones , amines etc. Rearrangement proceeds through sterically hindered six membered transition state. 33 Grace College Of Pharmacy

34. Fragmentation modes 34 Grace College Of Pharmacy

35. APPLICATIONS Molecular mass determination Isotopic abundance determination. Structural elucidation of organic and biologic molecules. Detection of impurities. Drug metabolism studies Quantitative analysis of mixtures To detect the distinction between cis and trans isomer Determination of ionization potential Detection of bonding informations . Clinical, toxicological and forensic applications. 35 Grace College Of Pharmacy

36. References Mass Spectrometry, Second edition by James Barker Elementary organic spectroscopy by Y.R. Sharma 36 Grace College Of Pharmacy

37. 37 Grace College Of Pharmacy