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Ionization Techniques – Introduction & Electron Ionization

CU- Boulder

CHEM 5181

Mass Spectrometry & Chromatography

Prof. Jose-Luis Jimenez

High Vacuum

Sample Inlet

Ion Source

MassAnalyzer

Detector Recorder

MS Interpretation

Lectures

Props: ionizer, filament

One slide adapted from Dr. Joel Kimmel’s 2007 lectureLast updated: Nov 2013

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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?

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“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).

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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?

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Ionization Methods Characteristics

From Lambert

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Effect of Ionization TechniquesFrom Schewdt

• 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

A B

C

D

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Electron Ionization Source Scheme

Electron Collector

ElectronEmissionFilament

-70 V

+

ExtractionPlate

-300 V

Ground0 V

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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 knowFromWatson

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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

• Ionization Efficiency

• What characteristic of the electron can we change to try to improve the results of ionization?

present molecules

formed ionsIE

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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

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Ionization Energy (M + e- → M+ + 2e-)

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

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Electron Affinity (M + e- → M-)

http://www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/electron-affinity.html

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EI Simulation w/ “Bohr” H AtomIncoming electron is scattered

Orbiting electron is excited

Ionization: both electrons escape

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

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The Concept of Cross Section

• E.g. “the 70 eV EI cross section of H2O is 2.3 A2”

• Electrons are coming perpendicular to page

PhysicalCross Section

ScatteringCross Section

70 eV Ionization

Cross Section

15 eV Ionization

Cross Section

Low Energy

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/

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Ionization Efficiency vs. Electron EnergyFrom Watson From de Hoffmann

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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

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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

• Frank-Condon principle: nuclei remain frozen in position

Log(t)10-16 s >10-12 s 10-5 s

molec

e

+

ee

frag1 frag2+

frag1+

“PostSourceDecay”

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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

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Energy Balance of EI (eV)

• Q: is energy conserved?A. Yes B. NoC. Partially D. I don’t know

molecule

e Ekin = 70

Eint = 0Ion +

e

e

Ekin = 55

Ekin = 0.1

EKin +Eint = 4.9

Before EI Ionization

After EI Ionization

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Internal Energy Distribution after EI

FromLambert

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“Soft” and “Hard” Events 1

From Lambert

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“Soft” and “Hard” Events 2

From Lambert

See also: http://schwinger.caltech.edu/~carl/ionization.html

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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?

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EI: Fragmentation vs. Electron Energy

• Clicker Q:• Is operation at

low electron energy desirable, in order to get better information at high m/z?

A. YesB. NoC. Doesn’t matterD. Not sure

FromHoffmann

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EI Fragmentation vs. Molecular Structure

FromLambert

Q: why is there such a dramatic difference in the spectra?A. Different amount of energy absorbedB. Different molecular stabilityC. Different propensity to tunnelingD. Different oxidation state E. Dunno

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Breakdown Curve for 1-propanol+

• This information can be precisely determined using electrons of a single energy, and scanning the energy

• This is what is different between molecules (prev. slide)

FromLambert

CH3-CH2-CH2OH+

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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?

1.

2.

3.

4.

60

6029

29 31

29 31 60

A. All of themB. None of themC. Only #2D. Only #1 & #3E. I don’t know

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Breakdown Curve

&Internal Energy

Distribution of Molecular

Ions I

FromLambert

Chemical Ionization

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FromLambert

Breakdown Curve

&Internal Energy

Distribution of Molecular

Ions II

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Breakdown Curve

+Internal Energy

Distributionof Molec. Ions IIIFrom

Lambert

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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

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(mainlib) 1-Propanol10 20 30 40 50 60 70

0

50

100

15

29

31

42

45

59

OH

Reproducibility: 4 Spectra of 1-Propanol in NIST

Again note that it doesn’t correspond to 70 eVs

of internal energy

(mainlib) 1-Propanol10 20 30 40 50 60 70

0

50

100

15

29

31

42

45

59

OH

(m ainlib) 1-Propanol10 20 30 40 50 60 70

0

50

100

15

29

31

42

45

59

OH

(replib) 1-Propanol10 20 30 40 50 60 70

0

50

100

2729

31

33

42 59

OH

(replib) 1-Propanol10 20 30 40 50 60 70

0

50

100

27

31

42

45 57

59

OH

Contributor: From Japan AIST/NIMC Database

Contributor: Chemical Concepts

Contributor: NIST Mass Spectrometry Data Center, 1990.

Contributor: also from NIST

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Advantages + Disadvantages of EI

From Lambert

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Appendix: Filament Emission Characteristics

The Mass Spec Handbook of Service.Published by Scientific Instrument Services, Inc.http://www.sisweb.com/

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Appendix II: Filament Burnout Patterns

MS Handbook Of Service