Mass Spectrometry (MS)

Transcription

1 Kevin Burgess, February 20, Mass Spectrometry (MS) from chapter(s) in the recommended text A. Introduction

2 Kevin Burgess, February 20, B. Components f Mass Spectrometers mass-to-charge. molecular mass without fragmentation). create ions in the gas phase separate ions on the basis of m/z (ie an analyzer) detect the number of ions of each m/z an ionization source / an analyzer / a detector. ionization. for analysis. or analysis. for analysis. or ionization. Fourier transform (FT) analysis. Electron Impact (EI) ionization. Detectors in MS electron multipliers. MALDI-TF is a valid description but ESI-MALDI is not. C. Primary Ions Formed In Different Ionization Techniques _MALDI_ ie [M + 1] +. _ESI_ [M + 1] +

3 Kevin Burgess, February 20, MALDI ESI EI ionization ionization N N N N ionization an electron from molecules to give radical cations ie [M] +. Resolution sensitivity. most sensitive forms of MS.

4 Kevin Burgess, February 20, D. Isotopes In Mass Spectrometry Element Isotope Abundance (%) Mass number Exact mass hydrogen carbon carbon nitrogen oxygen fluorine phosphorus sulfur chlorine chlorine bromine bromine iodine C C N F P S Br Br I C C 2 4 C exact mass exact mass exact mass C 3 79 Br C 3 81 Br C exact mass exact mass exact mass can distinguish

5 Kevin Burgess, February 20, atomic mass units (amu s) in a ratio of 3.13: amu s in a ratio of 1.03:1. 3 molecular ions in a 1:2:1 4 molecular ions in a 1:3:3:1 Illustrative Interpretation f Isotopes In MS the chlorine-containing compound A is number: 2 the bromine-containing compound B is number: 1 the non-halogenated compound C is number: 3 1 (m/z = 170 and 172):

6 Kevin Burgess, February 20, (m/z = 112 and 114): 3 (m/z = 107): always have odd molecular ion m/z values.

7 Kevin Burgess, February 20, E. Fragmentation radicals the most stable one that is most likely to be observed. electron impact ESI does not. ESI is widely used in contemporary MS, but EI Fragmentation is usually undesirable is useful when complementary methods MS/MS tandem C 2 6 C3

8 Kevin Burgess, February 20,

9 Kevin Burgess, February 20,

10 Kevin Burgess, February 20, C 2 5 a-eavage - 2

11 Kevin Burgess, February 20, of C-

12 Kevin Burgess, February 20, fragmentation fragmentation fragmentation Me fragmentation Me

13 Kevin Burgess, February 20, N 2 N 2 N 2 N N N N N N NEt 3 N

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15 Kevin Burgess, February 20, The McLafferty Rearrangement γ β α β β α α α α α β γ γ β α β β β a and b fragments McLafferty rearrangement alkene radical cation McLafferty rearrangement alkene radical cation McLafferty rearrangement

16 Kevin Burgess, February 20, McLafferty rearrangement McLafferty rearrangement McLafferty rearrangement Molecule on the left can undergo McLafferty rearrangement since it has g-hydrogen whereas molecule on the right does not.