Calculation of approximate intensity distribution due to
natural isotope abundance
Organic Structural Spectroscopy by Lambert, Shurvell, Lightner
Organic Structural Spectroscopy by Lambert, Shurvell, Lightner
Molecular formula determination by exact mass measurements
Examples
The calculated isotope ratios for CO, N2, and C2H4, all of molar mass 28, are as
follow:
CO
100 (M) 1.14 (M+1) 0.2 (M+2)
N2
C2H4
100 (M)
100 (M)
0.76 (M+1)
2.26 (M+1)
0.01 (M+2)
The calculated isotope ratios for C3H6 and CH2N2, both of molar mass 42, are as
follow:
C3H6
CH2N2
100 (M)
100 (M)
3.39 (M+1)
1.87 (M+1)
0.05 (M+2)
0.01 (M+2)
Calculated isotopic distribution for protonated bovine insulin. The chemical
molecular weight based on elemental atomic weights is 5734.6 Da. The
molecular weight based on the most abundant isotope of each element is
5728.6 Da, and the molecular weight based on the most abundant isotopic
form of the molecule is 5733.6 Da.
Different isotope distributions that match
the molecule mass 5733.6 Da
Schematic Representation of a Sector Mass Spectrometer
Spectroscopic Methods in OC by Hesse, Meier, Zeeh
Mass Analysis for Magnetic Sector MSs
Magnetic sectors deflect accelerated beams of ions, with the degree of
deflection depending on the mass, the charge, and the velocity of the ion beam.
The speed of ions is described by: v = (2zU / m)1/2
z = ionic charge, m = ion mass, U = acceleration potential (V)
The mass separation is given by: rm = mv / zB (cm)
B = magnetic field strength (T)
Both equations can be combined to give the fundamental equation of MS:
m/z = B2 rm2 / 2U
U and B are kept constant: m/z = constant rm2
U and rm are kept constant: m/z = constant B2
For typical values of Bmax = 1.5 Tesla, rm = 30 cm, and U = 5 kV, the
maximum value for m/z is 1953 Da/charge;
Metastable ions are generated by fragmentation after acceleration but before
entering the magnetic sector. The position of their diffuse peaks in the mass
spectrum is given by:
m* = m22/m1 z1/z22
Example: 1,2,3,4-tetrahydrocarbazole (M = 171) looses ethylene and a
metastable peak at 1432/171 = 119.6 is observed;
Fragmentation Patterns in EI-MS
The fragmentation in EI-MS involves exclusively gas phase unimolecular reactions
and is governed by product ion stability.
The relative stabilities of different ions are based on the same chemical models
that are also applicable in solution:
• Maintenance of an octet of electrons
• Localization of charge on the most favourable group/atom
• Resonance delocalization
• Absence of unpaired electrons
charge delocalization
Odd- and even-electron molecular and fragment ions
Odd+• can fragment into [even+ + neutral•] or [odd+• + neutral]
while even+ fragments into [even+ + neutral] but unlikely into [odd+• + neutral•]
Exceptions are found for even-electron ions containing particularly week
bonds, such as multiple C-Br bonds that undergo successive Br• losses,
or when particular stable odd-electron ions are formed.
Stevenson’s Rule for retro Diels-Alder and elimination reactions
IE = 9 eV
more
less
IE = 10 eV
• product ion enthalpy governs the dissociation pathway
• the fragment with the lowest ionization energy will preferentially take the charge
• the difference in activation energy equals the difference in IE (ionization energy)
Rearrangement versus simple cleavage
• rearrangements are intrinsically slower processes than simple bond
cleavages since they involve conformational changes
• rearrangements can only compete with bond cleavages if their activation
energies and the overall internal ion energies are low
• more than one new bond can be formed under low-energy conditions
(example to the right)
Charge localization
The two molecular radical ions shown above differ in internal energy by
about 1.2 eV due to their different IEs. The more electropositive N-atom
has the lower IE, therefore the higher internal energy, and is more likely to
fragment
α-cleavage of an oxygen ether and hetero-bond cleavage of the more polarizable
and electropositive thio ether
Remote fragmentations are possible and usually occur via a six-centred transition
state (see rearrangements).
Other terms
Ions in which the charged site is formally separated from the radical site are called
distonic ions.
Background noise (minor signals at almost every mass) is caused by ions of high
internal energy that undergo random bond cleavage.
Always Remember
No strict rules apply for mass spectral fragmentation, just as they are not for
describing product yields in solution phase reactions.