Mass Spectrometry

1. Mass Spectrometry

3. Mass Spectrometry • Principles • Mass Spectra http://www.mrc-dunn.cam.ac.uk/facilities/mass_spectrometry.php

4. Principles The study of ionised molecules in the gas phase • Principles date back to ~ 1897 • Based on accelerating ions in a vacuum • Joseph J. Thompson used an early MS to discover the electron . Received the Nobel Prize in 1906 [Physics]. For a “friendly” practical introduction see: http://www.asms.org/whatisms/p1.html and following pages USES • Molecular weight determination • Structural characterisation • Gas phase reactivity study • Qualitative and Quantitative analysis of mixtures

5. Sample Volatilisation Ionisation Separation Detection Principles • Volatilisation • Ionisation • Separation • Detection

6. Volatilisation • Gaseous and volatile samples are readily drawn into the reservoir. • Less volatile solid samples require heating. • The volatile sample diffuses from the reservoir into the ionization chamber via a leak - a pin-hole restriction in a gold foil.

7. Ionisation (1) Electron impact or EI (most common) • Sample passes through an electron beam. • energy of e- beam is increased until e- is ejected from the target molecule (normally ~70eV) • high energy causes substantial fragmentation M + e  [M]+. + 2e Radical cation [MOLECULAR ION] Most MS are set up to detect positive ions.

8. Ionisation (2) Chemical ionisation (CI) • Softer than EI (used for more sensitive compounds). • Sample is introduced with an excess of a carrier gas (eg NH3) which is ionised by the electron beam. • Lower energy means less fragmentation

9. Separation • A stable controllable magnetic field separates the ions according to their momentum. • Only ions of a single mass/charge [m/z] ratio will have the trajectory to be detected. • By varying the magnetic, field ions with different m/z values are focused on the detector

10. Detection • The ion current will cause emission of secondary electrons from a metal plate detector. • The Faraday cup detector suppresses secondary ion formation. • The -ve plate also suppresses secondary ion formation

11. Mass Spectra • Features • Fragmentation patterns

12. Features After ionisation: • Ejection of an electron from the parent molecule gives the molecular ion (generally M+, but depends on ionisation technique) • M+ fragments giving rise to other peaks Gives rise to a mass spectrum: • Plotted as intensity (%) vs m/z (amu) • The tallest peak is termed the base peak and is given an intensity of 100%. • Other peak intensities are given relative to this

13. Features

14. Fragmentation patterns (1) Occurs via two major routes: [M]+.  A+ (even electron cation) + B. (radical) OR [M]+.  C+. (cation radical) + D (molecule) NB Only particles with positive charges are detected

15. Fragmentation patterns (2) Characteristic fragmentation patterns arise as: • Weak bonds tend to break • Formation of stable fragments (ions, radicals and molecules) is favoured • Some molecules can form cyclic transition states

16. Fragmentation patterns • Alkanes • Alkenes and Aromatic compounds • Heteroatoms • Isotopes

17. Alkanes (1) • Carbocation is an sp2 hybrid with an empty p orbital. • Stability of the cation depends on the number of alkyl groups, due to inductive effects

18. Alkanes (2) Aliphatic carbon skeletons are readily cleaved at branches because it results in carbocations that are more stable

19. Alkanes (3) Stable fragments formed from branched alkyl chain results in less strong Molecular Ion (MI) NB Size of peak is relative

20. Alkenes and Aromatic compounds (1) • Cleavage occurs β to double bonds because more stable carbocations are produced due to resonance stabilisation • Cyclic alkenescan undergo rearrangements • Double bonds easily migrate in MS and isomers may easily be difficult to identify

21. Alkenes and Aromatic compounds (2) Aromatic hydrocarbons • strong molecular ion • βcleavage favoured strongly by resonance stabilisation • fragmentation of ring energetically disfavoured

22. Alkenes and Aromatic compounds (3) Aromatic hydrocarbons • Characteristic m/z = 91 and 65 from tropylium ion and breakdown product. Alkynes – strong MI, similar to alkenes, cleave at β site

23. Heteroatoms (1) Cleavage may occur α,β or γ to heteroatoms depending on the functional groups involved. BEWARE: α to the carbonyl is β to the heteroatom! a) αcleavage promoted by electronegativity eg ethers

24. Heteroatoms (2) b) β cleavage – promoted by resonance stabilisation eg Carbonyl group NB the most stable carbocation will be found in greatest abundance

25. Heteroatoms (3) c) McLafferty rearrangements give γ cleavage (β to carbonyl) • occurs with carbonyl containing compounds • depends on six membered transition state (i) Ketones, and carboxylic acids 1º carboxylic acids give a characteristic peak at m/z = 60

26. Heteroatoms (4) ii) Carboxylic esters Can undergo two types of McLafferty rearrangement

27. Heteroatoms • Oxygen heteroatoms • Nitrogen heteroatoms • Halogen heteratoms

28. Oxygen heteroatoms

29. Nitrogen heteroatoms • Weak or absent MI • iminium ion (like acylium) gives most intense peak • may also undergo cyclic rearrangements. Nitrogen rule: Compounds with an odd number of nitrogens have an odd molecular weight. eg NH3 MW = 17; CH3CH2NH2 MW = 45; NH2CH2CH2NH2 MW = 60

30. Halogen heteroatoms • MI is strongest for Iodides less so for Bromides etc. • More branched halides have weaker MIs. • Cl and Br Isotopes give visible patterns Fragmentation may include (in order of likelihood): • Loss of halogen • Loss of HX • βcleavage (as for oxygen) – loss of CH2X • Rearrangements

31. Isotopes (1) It is important to use the correct method to calculate molecular weight for MS. Average atomic masses take into account the different abundances of isotopes. Need to use exact mass for MS. CARBON 1.1% of naturally occurring carbon is 13C: gives 1% abundance m/z = 17 signal (M+H)+ in spectrum of Methane. HALOGENS Significant amounts of different isotopes, visible patterns within spectrum.

32. M Relative abundance M+2 Isotopes (2) Chlorine: Average Atomic weight = 35.453

33. M M+2 M+2 Relative abundance M M+4 Relative abundance m/z Isotopes (3) Bromine: Average atomic weight = 79.904 eg CH3Br Mass: 50% CH379Br = 94 50% CH381Br = 96 CH2Br2 Mass: 25 % CH279Br79Br = 172 25 % CH279Br81Br = 174 25 % CH281Br79Br = 174 25 % CH281Br81Br = 176 m/z Isotopic pattern for one bromine atom Isotopic pattern for one bromine atom

34. Characteristic signals in MS