Handout Ch3f2 Mass Spectrometry Barrow Lecture 4 2023-2024 PDF

CH3F2 (Advanced Analytical Chemistry) Mass Spectrometry: Tandem mass spectrometry Dr. Mark P. Barrow Fragmentation Electron ionization: fragmentation 2.4 Internal Energy and the Further Fate of Ions Fig. 2.7 Diagram showing the relative energy levels of species involved in ionization of methane and...

CH3F2 (Advanced Analytical Chemistry) Mass Spectrometry: Tandem mass spectrometry Dr. Mark P. Barrow Fragmentation Electron ionization: fragmentation 2.4 Internal Energy and the Further Fate of Ions Fig. 2.7 Diagram showing the relative energy levels of species involved in ionization of methane and loss of an H from the molecular ion. Values are given in kJ mol–1 and rounded to integer numbers 45 ~1.8 eV ~12.6 eV ~13.4 eV molecular ions proceeding by formation of secondary or tertiary radicals and/or ions to become dominant over those leading to smaller and/or primary radical and ionic fragments, respectively (Sect. 6.2). “Mass Spectrometry: A textbook,” Jürgen Gross, Springer, ISBN: 978-3-319-54397-0 + 70 eV for EI is higher than ionization energy for molecule, additional energy results in fragmentation Electron ionization: fragmentation Isomers: branching “Mass Spectrometry: A textbook,” Jürgen Gross, Springer, ISBN: 978-3-319-54397-0 Isomers: double bond positions Electron ionization: fragmentation Odd electron ions are radical ions; they have an unpaired electron Even electron ions have paired electrons “Mass Spectrometry: A textbook,” Jürgen Gross, Springer, ISBN: 978-3-319-54397-0 Tandem mass spectrometry Intentional fragmentation used to obtain structural information “Tandem mass spectrometry” or “mass spectrometry/mass spectrometry” (MS/MS) Commonly involves an “isolation” step No isolation: fragmentation of all species observed in the same experiment; complex spectra and cannot determine which fragment came from which precursor Isolation: ion of interest is selected (e.g. using a quadrupole) for fragmentation and the relation between precursor and fragments is known MSn if performing multiple steps (e.g. fragmentation of a fragment from an earlier step) Methods of tandem mass spectrometry "Mass Spectrometry," James McCullagh and Neil Oldham, Oxford Chemistry Primers, ISBN: 9780198789048 Methods of tandem mass spectrometry “Mass Spectrometry: A textbook,” Jürgen Gross, Springer, ISBN: 978-3-319-54397-0 Collisioninduced dissociation Collision-induced dissociation (CID) Ions collide with neutral gases and dissociate For a given choice of gas, energy of the collision is controlled by changing the kinetic energy of the ion Fragments scatter radially Slow fragmentation method, deposits vibrational energy throughout the ion prior to fragmentation Most common MS/MS technique Keith R. Jennings (At Warwick: 1972-1995) Collision-induced dissociation (CID) "Mass Spectrometry," James McCullagh and Neil Oldham, Oxford Chemistry Primers, ISBN: 9780198789048 CID: centre-of-mass collision energy where Ecom is the centre-of-mass collision energy, mN is the mass of the neutral gas, mp is the mass of the precursor ion, and Elab is the translational energy of the ion For a given ion, Ecom therefore depends upon kinetic energy of ion (selected by the user) and the mass of the chosen collision gas Elab = q V = z e V where V is the potential (e.g. accelerating potential) used, z is the number of charges on the ion, and e is the elementary charge Note: kinetic energy of ion proportional to charge when it is accelerated "Mass Spectrometry," James McCullagh and Neil Oldham, Oxford Chemistry Primers, ISBN: 9780198789048 Triple quadrupole https://commons.wikimedia.org/wiki/User:Fulvio314 m/z = x Scan for m/z = x - y "Mass Spectrometry," James McCullagh and Neil Oldham, Oxford Chemistry Primers, ISBN: 9780198789048 TOF Laser Source deflector Vs + + + first field free drift region Delay Generator Vr ≈ Vs ++ ++ + + Collision Cell (Vc) + + + + second field free drift region Detector Oscilloscope Q-TOF "Mass Spectrometry," James McCullagh and Neil Oldham, Oxford Chemistry Primers, ISBN: 9780198789048 MSn MS For example, FT-ICR experiments MS/MS "Mass Spectrometry," James McCullagh and Neil Oldham, Oxford Chemistry Primers, ISBN: 9780198789048 Interpretation Interpretation Consider the experiment What ionization technique was used? o This influences the species formed during ionization, whether even-electron or odd-electron (e.g. [M + H]+ or M+ ) What dissociation technique was used? o This influences the product ions formed when dissociation takes place Assign elemental compositions to the product ions If you have a proposed structure of the analyte, look at the possible weak-points that may break “Curly arrows” Use of “curly arrows” to represent movement of electrons Heterolytic cleavage Two electrons moved to the same atom Homolytic cleavage One electron moved to each of the two atoms Electron counting AB+ Odd electron ion AB+ Even electron ion A + + B A+ + B A+ + B A+ + B Even electron ion Odd electron ion Even electron ion Odd electron ion Precursor ion (high energy) results in fragments of lower energy Dissociation mechanisms Dissociation is affected by: Inductive effects Resonance effects Product ion stability Inductive effects X-C+ R X-C+ R Stabilizing effect Destabilizing effect (electron-donating groups) (electron-withdrawing groups) R = alkyl groups R = -NO2 -CN -COOH, -COOR -OH, -OR -CHO, -COR -F, -Cl, -Br, -I -C6H5, -CH=CH2 -SO3H -SH, -SR -NH2 Resonance effects X-C+ Stabilizing effect (electron donating groups) R= -alkyl -F,-Cl, -Br, -I -OH, -OR -C6H5, -CH=CH2 -SH, -SR -NH2 R Destabilizing effect (electron withdrawing groups) R= -NO2 -CN -CHO, -COR -COOH, -COOR -SO3H Examples of effects Inductive effect Resonance effect Carbocation stability More stable CH3 CH3 CH+ CH3 CH3 CH+ CH3 CH3 CH2+ CH3+ H+ Less stable Common observations Aliphatic compounds Cyclic structures Straight cleavages Rearrangements OH, C=O groups SO, CN groups Rearrangements Rearrangements Amines Loss of NH3 Hydroxyl groups Loss of H2O Acid groups Loss of CO2 and HCOOH Aromatic systems Difficult to fragment Examples of rearrangement and cyclisation Often observed when using CID, loss of neutral molecule Can lead to stable product ions Other methods and biomolecules Photodissociation Ion absorbs photon(s) and dissociates Energy of the fragmentation is controlled by changing the photon’s wavelength (e.g. choice of laser) No scattering, except for multiply-charged ions Examples: infrared multiphoton dissociation (IRMPD) and ultraviolet photodissociation (UVPD) IRMPD: slow fragmentation method, deposits vibrational energy throughout the ion prior to fragmentation (depends upon wavelength) Electron capture dissociation (ECD) Multiply-charged ions capture a slow electron (e.g.