Chapter 2 - Principles of ionization and ion dissociation - part 1 Flashcards
Some neutrals may by chance hit the detector which contributes to
noise on the signal.
Electron impact (EI)
Shooting energetic electrons on a gaseous neutral.
EI predominantly creates ______ charged ions from the precursor neutral. (number of charge)
singly
Ionization of polycyclic aromatic hydrocarbons often gives a moderate abundance of
doubly-charged ions.
Penning ionization occurs with the (trace) gas M having an ionization energy
lower than the energy of the metastable state of the excited (noble gas) atoms (aka, it takes less energy required to remove the electron than the energy that is added to the molecule).
Neutrals in EI are formed by
excitation.
Cations in EI are formed by
Ionization
Dissociative ionization
Dissociative rearrangement
Anions in EI are formed by
Electron capture
Dissociative electron capture
Ions of both polarities in EI are formed by
ion-pair formation.
Ionization energy
The amount of energy required to remove an electron from an atom or molecule in the ground state.
Molecules with lone pairs usually have a _____ ionization energy (higher or lower).
Lower
Ionization energies of most molecules are in the range of
7-15 eV
Removal of an electron from a molecule can formally be considered to occur at a
sigma-bond (least favored)
pi-bond
lone pair (most favored)
Born-Oppenheimer approximation
The assumption that electronic motions and nuclear motions can be separated due to the large mass difference between nuclei and electrons.
Franck-Condon principle
Electronic transitions will occur on a much faster timescale than it takes the nuclei to move to their new equilibrium positions.
Ions having an internal energy above the dissociation energy level will
dissociate at some point leading to causing fragment ions within a mass spectrum.
In EI, the electron energies are much higher than the ionization energies of molecules because
the odds of all the energy of the electron being transferred to the molecule is fairly low.
Degrees of freedom for diatomic or linear molecules.
s = 3N - 5
Degrees of freedom for non-linear molecules
s = 3n - 6
Appearance energy
The amount of energy needed to be transferred to the neutral M to allow for the detection of the fragment ion m1+.
Homolytic bond dissociation enthalpies give
the energy needed to cleave a bond of the neutral molecule to obtain a pair of radicals. Homolytic bond dissociation enthalpies range from 3-5 eV.
Heterolytic bond dissociation enthalpies give
the energy needed to cleave a bond resulting in the formation of a cation and anion. Heterolytic bond dissociation enthalpies range from 10-13 eV.
The heat of formation of organic radicals and positive ions decreases with ______ and _______.
their size, and more importantly, their degree of branching at the radical or ionic site (secondary and tertiary radicals are more favored).
A lower heat of formation is equivalent to a higher
thermodynamic stability of the respective ion or radical.
Quasi-equilibrium theory (QET)
A theoretical approach for describing the unimolecular decompositions of ions and hence their mass spectra.
Rice-Ramsperger-Kassel-Marcus (RRKM) theory
A chemical kinetics theory allowing estimation of unimolecular reaction rates from a few parameters of the potential energy surface.
According to QET the rate constant, k, of a unimolecular reaction is basically a function of
excess energy, Eex, of the reactants in the transition state and thus k(E) strongly depends on the internal energy distribution of any particular ion.
The rate constants of unimolecular reactions have the dimension
per second (s^-1)
Stable ions rate constant
k < 10^5 s^-1
Metastable ions rate constant
10^5 s^-1 < k < 10^6 s^-1
Unstable ions rate constant
k > 10^6 s^-1
Typical quadrupole ion flight time
57 microseconds
Typical magnetic sector ion flight time
45 microseconds
Typical time-of-flight ion flight time
45 microseconds
Fourier transform ion cyclotron resonance (FT-ICR) relies on
storage of ions on circular paths to detect the angular frequencies of coherent ion packages as they repeatedly pass a pair of detection plates.
Linear quadrupole ion trap (LIT)
Linear radiofrequency multipoles can be axially segmented or equipped with trapping plates to their ends to raise switchable potential walls that allow to accumulate, store, and eject ion packages as required.
Three-dimensional quadrupole ion trap (QIT)
Stores ion clouds to have them ready for mass analysis by bringing them successively from stable to unstable ion trajectories that end up with ejection from the trap onto a detector.
The time span of ion accumulation can easily extend from
milliseconds to seconds.
The passage between functional units of the instrument may take
tens of microseconds to milliseconds.
m/z analysis requires storage times from
milliseconds (QIT, LIT) to seconds (FT-ICR, Orbitrap).
For cases where ion storage is involved, there is a ________ fold expansion of the classical mass spectrometric time scale.
10^3-10^6
The handling of ions on the extended time scale is only possible if they are
stable (i.e., non-decomposing)
