2. The Nature of Mass Spectra
A mass spectrum will usually be presented as a vertical bar graph, in which
each bar represents an ion having a specific mass-to-charge ratio (m/z) and the
length of the bar indicates the relative abundance of the ion. The most intense
ion is assigned an abundance of 100, and it is referred to as the base peak.
Most of the ions formed in a mass spectrometer have a single charge, so the m/z
value is equivalent to mass itself. Modern mass spectrometers easily distinguish
(resolve) ions differing by only a single atomic mass unit (amu), and thus
provide completely accurate values for the molecular mass of a compound. The
highest-mass ion in a spectrum is normally considered to be the molecular ion,
and lower-mass ions are fragments from the molecular ion, assuming the sample is
a single pure compound.
The following diagram displays the mass spectra of three simple gaseous
compounds, carbon dioxide, propane and cyclopropane. The molecules of these
compounds are similar in size, CO2 and C3H8
both have a nominal mass of 44 amu, and C3H6 has a mass of
42 amu. The molecular ion is the strongest ion in the spectra of CO2
and C3H6, and it is moderately strong in propane. The unit
mass resolution is readily apparent in these spectra (note the separation of
ions having m/z=39, 40, 41 and 42 in the cyclopropane spectrum). Even though
these compounds are very similar in size, it is a simple matter to identify them
from their individual mass spectra. By clicking on each spectrum in turn, a
partial fragmentation analysis and peak assignment will be displayed. Even with
simple compounds like these, it should be noted that it is rarely possible to
explain the origin of all the fragment ions in a spectrum. Also, the structure
of most fragment ions is seldom known with certainty.
Since a molecule of carbon dioxide is composed of only three atoms, its mass
spectrum is very simple. The molecular ion is also the base peak, and the only
fragment ions are CO (m/z=28) and O (m/z=16). The molecular ion of propane also
has m/z=44, but it is not the most abundant ion in the spectrum. Cleavage of a
carbon-carbon bond gives methyl and ethyl fragments, one of which is a
carbocation and the other a radical. Both distributions are observed, but the
larger ethyl cation (m/z=29) is the most abundant, possibly because its size
affords greater charge dispersal. A similar bond cleavage in cyclopropane does
not give two fragments, so the molecular ion is stronger than in propane, and is
in fact responsible for the the base peak. Loss of a hydrogen atom, either
before or after ring opening, produces the stable allyl cation (m/z=41). The
third strongest ion in the spectrum has m/z=39 (C3H3). Its
structure is uncertain, but two possibilities are shown in the diagram. The
small m/z=39 ion in propane and the absence of a m/z=29 ion in cyclopropane are
particularly significant in distinguishing these hydrocarbons.
Most stable organic compounds have an even number of total electrons,
reflecting the fact that electrons occupy atomic and molecular orbitals in
pairs. When a single electron is removed from a molecule to give an ion, the
total electron count becomes an odd number, and we refer to such ions as
radical cations. The molecular ion in a mass spectrum is always a radical
cation, but the fragment ions may either be even-electron cations or
odd-electron radical cations, depending on the neutral fragment lost. The
simplest and most common fragmentations are bond cleavages producing a neutral
radical (odd number of electrons) and a cation having an even number of
electrons. A less common fragmentation, in which an even-electron neutral
fragment is lost, produces an odd-electron radical cation fragment ion. Fragment
ions themselves may fragment further. As a rule, odd-electron ions may fragment
either to odd or even-electron ions, but even-electron ions fragment only to
other even-electron ions.
The masses of molecular and fragment ions also reflect the electron count,
depending on the number of nitrogen atoms in the species.
Ions with
no nitrogen
or an even #
N atoms |
odd-electron
ions
even-number
mass |
even-electron
ions
odd-number
mass |
Ions having
an
odd # N
atoms |
odd-electron
ions
odd-number
mass |
even-electron
ions
even-number
mass |
This distinction is illustrated nicely by the follwing two examples. The
unsaturated ketone, 4-methyl-3-pentene-2-one, on the left has no nitrogen so the
mass of the molecular ion (m/z = 98) is an even number. Most of the fragment
ions have odd-numbered masses, and therefore are even-electron cations.
Diethylmethylamine, on the other hand, has one nitrogen and its molecular mass
(m/z = 87) is an odd number. A majority of the fragment ions have even-numbered
masses (ions at m/z = 30, 42, 56 & 58 are not labeled), and are even-electron
nitrogen cations. The weak even -electron ions at m/z=15 and 29 are due to
methyl and ethyl cations (no nitrogen atoms). The fragmentations leading to the
chief fragment ions will be displayed by clicking on the appropriate spectrum.
Repeated clicks will cycle the display.
4-methyl-3-pentene-2-one |
|
N,N-diethylmethylamine |
When non-bonded electron pairs are present in a molecule (e.g. on N or O),
fragmentation pathways may sometimes be explained by assuming the missing
electron is partially localized on that atom. A few such mechanisms are shown
above. Bond cleavage generates a radical and a cation, and both fragments often
share these roles, albeit unequally.
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