Chapter 2 - Principles of ionization and ion dissociation - part 1

Question 1

Some neutrals may by chance hit the detector which contributes to

Answer 1

noise on the signal.

Question 2

Electron impact (EI)

Answer 2

Shooting energetic electrons on a gaseous neutral.

Question 3

EI predominantly creates ______ charged ions from the precursor neutral. (number of charge)

Answer 3

singly

Question 4

Ionization of polycyclic aromatic hydrocarbons often gives a moderate abundance of

Answer 4

doubly-charged ions.

Question 5

Penning ionization occurs with the (trace) gas M having an ionization energy

Answer 5

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).

Question 6

Neutrals in EI are formed by

Answer 6

excitation.

Question 7

Cations in EI are formed by

Answer 7

Ionization
Dissociative ionization
Dissociative rearrangement

Question 8

Anions in EI are formed by

Answer 8

Electron capture

Dissociative electron capture

Question 9

Ions of both polarities in EI are formed by

Answer 9

ion-pair formation.

Question 10

Ionization energy

Answer 10

The amount of energy required to remove an electron from an atom or molecule in the ground state.

Question 11

Molecules with lone pairs usually have a _____ ionization energy (higher or lower).

Answer 11

Lower

Question 12

Ionization energies of most molecules are in the range of

Answer 12

7-15 eV

Question 13

Removal of an electron from a molecule can formally be considered to occur at a

Answer 13

sigma-bond (least favored)
pi-bond
lone pair (most favored)

Question 14

Born-Oppenheimer approximation

Answer 14

The assumption that electronic motions and nuclear motions can be separated due to the large mass difference between nuclei and electrons.

Question 15

Franck-Condon principle

Answer 15

Electronic transitions will occur on a much faster timescale than it takes the nuclei to move to their new equilibrium positions.

Question 16

Ions having an internal energy above the dissociation energy level will

Answer 16

dissociate at some point leading to causing fragment ions within a mass spectrum.

Question 17

In EI, the electron energies are much higher than the ionization energies of molecules because

Answer 17

the odds of all the energy of the electron being transferred to the molecule is fairly low.

Question 18

Degrees of freedom for diatomic or linear molecules.

Answer 18

s = 3N - 5

Question 19

Degrees of freedom for non-linear molecules

Answer 19

s = 3n - 6

Question 20

Appearance energy

Answer 20

The amount of energy needed to be transferred to the neutral M to allow for the detection of the fragment ion m1+.

Question 21

Homolytic bond dissociation enthalpies give

Answer 21

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.

Question 22

Heterolytic bond dissociation enthalpies give

Answer 22

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.

Question 23

The heat of formation of organic radicals and positive ions decreases with ______ and _______.

Answer 23

their size, and more importantly, their degree of branching at the radical or ionic site (secondary and tertiary radicals are more favored).

Question 24

A lower heat of formation is equivalent to a higher

Answer 24

thermodynamic stability of the respective ion or radical.

Question 25

Quasi-equilibrium theory (QET)

Answer 25

A theoretical approach for describing the unimolecular decompositions of ions and hence their mass spectra.

Question 26

Rice-Ramsperger-Kassel-Marcus (RRKM) theory

Answer 26

A chemical kinetics theory allowing estimation of unimolecular reaction rates from a few parameters of the potential energy surface.

Question 27

According to QET the rate constant, k, of a unimolecular reaction is basically a function of

Answer 27

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.

Question 28

The rate constants of unimolecular reactions have the dimension

Answer 28

per second (s^-1)

Question 29

Stable ions rate constant

Answer 29

k < 10^5 s^-1

Question 30

Metastable ions rate constant

Answer 30

10^5 s^-1 < k < 10^6 s^-1

Question 31

Unstable ions rate constant

Answer 31

k > 10^6 s^-1

Question 32

Typical quadrupole ion flight time

Answer 32

57 microseconds

Question 33

Typical magnetic sector ion flight time

Answer 33

45 microseconds

Question 34

Typical time-of-flight ion flight time

Answer 34

45 microseconds

Question 35

Fourier transform ion cyclotron resonance (FT-ICR) relies on

Answer 35

storage of ions on circular paths to detect the angular frequencies of coherent ion packages as they repeatedly pass a pair of detection plates.

Question 36

Linear quadrupole ion trap (LIT)

Answer 36

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.

Question 37

Three-dimensional quadrupole ion trap (QIT)

Answer 37

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.

Question 38

The time span of ion accumulation can easily extend from

Answer 38

milliseconds to seconds.

Question 39

The passage between functional units of the instrument may take

Answer 39

tens of microseconds to milliseconds.

Question 40

m/z analysis requires storage times from

Answer 40

milliseconds (QIT, LIT) to seconds (FT-ICR, Orbitrap).

Question 41

For cases where ion storage is involved, there is a ________ fold expansion of the classical mass spectrometric time scale.

Answer 41

10^3-10^6

Question 42

The handling of ions on the extended time scale is only possible if they are

Answer 42

stable (i.e., non-decomposing)