Ionization Techniques – Introduction & Electron Ionization

CU- Boulder CHEM 5181 Mass Spectrometry & Chromatography MS Interpretation Prof. Jose-Luis Jimenez Lectures

High Vacuum

Sample Ion Mass Detector Recorder Inlet Source Analyzer

One slide adapted from Dr. Joel Kimmel’s 2007 lecture Last updated: Nov 2013 Props: ionizer, filament 1

Business Items • Midterm distributed @ end of class – See Piazza for details – Added ranking for your reference, no effect on grade – Questions should be addressed during office hours or in Piazza • Questions or comments?

2 “The development of mass spectrometry can be seen, from one perspective, to be based on the invention and utilization of ion sources of ever greater power and more general applicability.”

- R. G. Cooks et al., J. Mass Spec, 2005, 1261

•Early stages of MS: Precise determination of atomic masses and isotope abundances. •Last 50 years: Shift towards analytical applications involving molecules of greater complexity •Last 20 years: Explosion of biological applications

Recommended reading: Vestal, Chem. Rev., 101, 361, 2001 (on Wiki). 3

Some Commonly Used Ionization Techniques • Electron Ionization (EI) • Chemical Ionization (CI) • Electrospray (ESI) / Nanospray • Desorption Techniques – Matrix-Assisted Laser Desorption/Ionization (MALDI) – Fast Atom Bombardment (FAB) • Ambient: DESI / DART • Ionization for Elemental Analysis – Thermal Ionization Source – Spark Source – Glow Discharge – Inductively-Coupled Plasma (ICP)

Q: why are so many ionization techniques used in MS? 4 Ionization Methods Characteristics

From Lambert 5

From Schewdt Effect of Ionization Techniques

A B

D

C

• Same molecule analyzed by 4 techniques • Clicker Q: which is preferable: A, B, C, D, or E: it depends • Goal for today: understand why this happens 6 Electron Ionization Source Scheme

Extraction Plate -300 V

Ground 0 V +

Electron Emission Filament -70 V Electron Collector 7

Electron Ionization Source

• Clicker Q: why are there magnets on the EI sourced I passed around? A. To increase ionization efficiency B. To increase electron path C. To increase ion transmission to MS D. To increase filament lifetime E. I don’t know From Watson

8 EI Notes 1

• Hot filament giving off electrons – “Thermoionic effect” – W or Re filament • Accelerated by a potential difference towards and anode • Interact with the gaseous molecules in their path – Do not “impact” them ions formed • Ionization Efficiency IE  molecules present • What characteristic of the electron can we change to try to improve the results of ionization?

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Reference Energy Values & Conversions • 1 eV ~ 23 kcal/mol = 96 kJ/mol • Average bond energies (kcal/mol): – C-C: 83 C-O: 86 C-H: 99 C-N: 73 – C-Cl: 81 C-Br: 68 O-O: 35 H-F: 135 – C=C: 146 C=O: 178 C=N: 147 –C≡C: 200 C≡N: 213 N≡N: 226 – There is variability, e.g. dissociation of C-H in: • Methyl: 103 Ethyl: 98 Benzyl: 85 • Phenyl: 110 Vinyl: 112

•Energy unit conversion: http://mukamel.ps.uci.edu/~zyli/science/units.html •Bond energies: http://www.cem.msu.edu/~reusch/OrgPage/bndenrgy.htm 10 Ionization Energy (M + e- → M+ + 2e-)

http://en.wikipedia.org/wiki/Ionization_energy 11

Electron Affinity (M + e- → M-)

http://www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/electron-affinity.html 12 EI Simulation w/ “Bohr” H Atom

Incoming electron is scattered Ionization: both electrons escape

Orbiting electron is excited Replacement of orbiting w/ incoming e-

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Electron Interaction Cross Sections (SF6)

http://www.eeel.nist.gov/811/refdata/ http://physics.nist.gov/Divisions/Div842/Icamdata/PDF/1Databases/christo.pdf 14 The Concept of Cross Section

2” • E.g. “the 70 eV EI cross section of H2O is 2.3 A • Electrons are coming perpendicular to page

Physical Scattering 70 eV 15 eV Cross Cross Ionization Ionization Low Section Section Cross Cross Energy Section Section Attachment

(e.g. SF6)

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Electron Interaction Cross Sections (CF4)

http://physics.nist.gov/Divisions/Div842/Icamdata/PDF/1Databases/christo.pdf http://www.eeel.nist.gov/811/refdata/ 16 Ionization Efficiency vs. Electron Energy

From Watson From de Hoffmann

17

Clicker Q

- • With respect to e interaction ionization, SF6 and CF4 are: A. Similar for positive and negative ionization B. Similar for positive and different for negative C. Different for positive and similar for negative D. Different for positive and negative E. I don’t know

18 Time Scales of Ionization • What happens to the molecule when an electron goes by? – 70 eV electron => 5 x 106 m/s – Molecule = 10 A = 1 nm • Transit time = 2 x 10-16 s • Molecular vibrations > 10-12 s • Electronic time scale ~ 10-16 s e • Frank-Condon principle: nuclei remain frozen in position

+ + molec frag1+ frag2 frag1

“Post 10-16 s>10e -12 s10Source -5 s Log(t) e Decay” 19

EI Notes 3 • What electron energy would be most interactive with the molecule? • Each electron is associated with a wave •  = h / (mv) • 2.7 A for 20 eV, 1.4 A for 70 eV • Wave is “dispersed” into many frequencies. If one of them has an energy hv corresponding to an electronic transition in molecule, energy transfer leads to excited electronic state – 10 to 20 eV are transferred to the molecule. Only 10 eV are needed to ionize, so rest of the energy can lead to fragmentation • Ionization potential: energy it would take to eject the weakest bound electron from the molecule • At very high energies the wavelength becomes too small, and the molecules become “transparent” to the electron. In other words: not enough time to interact + transfer energy 20 Energy Balance of EI (eV)

Before EI Ionization

e Ekin = 70

After EI Ionization

E +E = 4.9 Eint = 0 Kin int molecule Ion +

Ekin = 0.1

e • Q: is energy conserved? e A. Yes B. No Ekin = 55 C. Partially D. I don’t know 21

Internal Energy Distribution after EI

From Lambert

22 “Soft” and “Hard” Events 1

From Lambert

23

See also: http://schwinger.caltech.edu/ ~carl/ionization.html “Soft” and “Hard” Events 2

From Lambert

24 Fragmentation notes • Fragmentation depends on: – Internal energy deposition on the ion • Shapes of the potential energy hypersurfaces • Energy of the interacting electrons – Molecular structure resists fragmentation • Chemical nature of the analyte • Is fragmentation good or bad?

25

EI: Fragmentation vs. Electron Energy

From • Clicker Q: Hoffmann • Is operation at low electron energy desirable, in order to get better information at high m/z? A. Yes B. No C. Doesn’t matter D. Not sure

26 From Lambert EI Fragmentation vs. Molecular Structure

Q: why is there such a dramatic difference in the spectra? A. Different amount of energy absorbed B. Different molecular stability C. Different propensity to tunneling D. Different oxidation state E. Dunno 27

Breakdown Curve for 1-propanol+

+ • This information CH3-CH2-CH2OH can be precisely determined using electrons of a single energy, and scanning the energy • This is what is different between molecules (prev. slide)

From Lambert 28 Clicker Q on Breakdown Curve

• Chemical ionization transfers a very well defined amount of energy to the analyte molecules. Which of the following spectra are possible with CI? 60 1. 29 60 2. 29 31 3. A. All of them B. None of them 29 31 60 4. C. Only #2 D. Only #1 & #3

E. I don’t know 29

Breakdown Curve & Internal Energy Distribution of Molecular Chemical Ionization Ions I

From Lambert 30 Breakdown Curve & Internal Energy Distribution of Molecular Ions II

From Lambert 31

Breakdown Curve + Internal Energy Distribution of Molec.

From Ions III Lambert 32 Electron Ionization Notes • Big advantage: high reproducibility of the fragmentation because – Purely physical (not chemical) process – Fragmentation involves only gas-phase unimolecular reactions • However all MS are far less reproducible than those based in the interaction of electromagnetic radiation with matter (IR, NMR) – MS depend on distribution of molecule internal energies, electron energies, time allowed for fragmentation, efficiency of spectrometer vs m/z 33

Reproducibility: 4 Spectra of 1-Propanol in NIST

31 Contributor: NIST 31 100 31 100 100 Mass Spectrometry Contributor: also from Data Center, 1990. Again noteNIST that it doesn’t correspond to 70 eVs

OH of internal energyOH 50 50

29 29 42 42 59 59 15 15 45 45 0 0 OH 10 20 30 40 50 60 70 10 20 30 40 50 60 70 50 (mainlib) 1-Propanol (mainlib) 1-Propanol

31 31 100 Contributor: Chemical 100 Contributor: From Concepts Japan AIST/NIMC Database 29 42 OH 50 OH 50 59 15 45 0 27 59 42 10 29 20 42 30 59 40 50 27 60 70 33 45 57 (mainlib)0 1-Propanol 0 10 20 30 40 50 60 70 10 20 30 40 50 60 70 34 (replib) 1-Propanol (replib) 1-Propanol Advantages + Disadvantages of EI

From Lambert 35

Appendix: Filament Emission Characteristics

The Mass Spec Handbook of Service. Published by Scientific Instrument Services, Inc. http://www.sisweb.com/ 36 Appendix II: Filament Burnout Patterns

MS Handbook Of Service

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