MASS SPECTROMETRY
The concept of MS
A compound is ionized (ionization
method)
 the ions are separated on the basis of
their mass/charge ratio (ion separation
method)
 the number of ions representing each
mass/charge (m/z) "unit" is recorded as
a spectrum

example: the mass spectrum of
benzamide (EI ionization mode)
Usage of MS



can be combined with other techniques (GCMS, LC-MS)
widespread use for studying known or unknown
compounds
identification of compounds


computer search of an MS library
in most cases very convincing evidence of the
identity of compound


the molecular ion, the fragmentation pattern
for identification of unknown compound combined data from
other spectroscopic techniques (IR and NMR) are required
Instrumentation

Resolution

the ability of an instrument to record the molecular
weight of the compound under examination to the
nearest whole number




to distinguish a peak at e.g. mass 300 from a peak at
mass 299 or 301
adjacent peaks must be cleanly separated
the valley between two such peaks should not be
more than 10% of the height of the larger peak
this degree of resolution is termed "unit" resolution

can be obtained up to a mass of approximately 3000 Da
on "unit resolution" instruments
Determining the resolution of an
instrument

to choose two peaks with the height of
the valley between them not exceeding
10% of the more intense peak
R=Mn/(Mn-Mm)
Low (unit) and high resolution MS
spectrometers

Low-resolution instruments:


able to separate unit masses up to m/z
3000 [R=3000/(3000-2999)=3000]
High-resolution instruments
R=20,000-100,000
 able to determine molecular formula

General scheme of an MS
Ionization methods.
Gas-phase ionization
the oldest and most popular method
 applicable to compounds which have a
minimum vapor pressure of ~10-6 Torr at
a temperature at which the compound is
stable
 a large number of non-ionic organic
molecules with MW<1000

Electron impact ionization (EI)






the most widely used method for generating ions for
MS
vapor phase sample molecules are bombarded with
high-energy electrons (generally 70 eV),
the electrons eject an electron from a sample molecule
to produce a radical cation (the molecular ion)
the ionization potential of typical organic compounds is
generally less than 15 eV
the bombarding electrons impart 50 eV (or more) to
the molecular ion
breaking covalent bonds (bond strengths 3-10 eV)
Electron impact ionization (cont.)




bond breaking is usually extensive and highly
reproducible; it is a characteristic of the
compound
fragmentation process is predictable and base
for structure elucidation
the excess energy imparted to the molecular
ion is often too high → the molecular ion can
not be observed
usual "cure": reducing the ionization voltage


fragmentation is reduced
disadvantage: no comparison with the standard
library spectra is possible
Chemical ionization (CI)


one of so called "soft ionization" techniques
sample molecules in vapor phase collide with ionized
reagent gas in the source





the pressure in CI source is relatively high
secondary ionization occurs by:



reagent gas: methane, isobutane, ammonia...
ionization of reagent gas is achieved by electron impact
ions of reagent gas: CH5+, C4H9+, etc.
proton transfer → [M+1]+ ion
electrophilic addition → [M+15]+ , [M+24]+ , [M+43]+ , or
[M+18]+ (with NH4+)
sometimes [M-1]+ ions are generated by hydride
abstraction
Chemical ionization (cont.)



the excess energy transferred to the sample
molecules is small (usually less than 5 eV)
less fragmentation occurs
advantages:



abundance of molecular ions
greater sensitivity (total ion current is concentrated
into a few ions)
disadvantages:


less information on structure (only few fragment
ions, sometimes none)
presence of very stable quasimolecular ions [M+1]+
Chemical ionization (cont.)
Chemical ionization (cont.)




[M+29]+ (M+C2H5+) and [M+41]+ (M+C3H5+) are
a result of electrophilic addition of
carbocations
useful in identifying the molecular ion
the electron impact ionization of CH4 gives
CH4+ and CH3+
with the excess of methane they give
secondary ions:
CH3+ + CH4 → C2H5+ + H2
C2H5+ + CH4 → C3H5+ + 2H2
Chemical ionization (cont.)

the energy content of the various secondary
ions (from methane, isobutane and ammonia):
CH5+ > t-C4H5+ > NH4+

the choice of the reagent gas – the control of
tendency of [M+1]+ ion to fragment
the main purpose of CI: to detect molecular
ions (to determine molecular weights)

Desorption ionization methods

Field desorption ionization (FD)

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
the sample is applied to a metal emitter
the surface of metal emitter is covered by carbon
microneedles
accelerating voltage is applied on of the surface
(functions as the anode)
very high voltage gradients at the tips of the
needles remove an electron from the sample
the resulting cation is repelled from the emitter
the generated ions have little excess energy – the
fragmentation is minimal

the molecular ion is usually the only significant ion seen
FD - advantages
very useful for nonpolar, low volatile
compounds
 no problems with the high level of
background ions, as in the matrixassisted fast atom bombardment
ionization method (FAB)

Fast atom bombardment ionization
method (FAB)




the method uses high energy xenon or argon
atoms (6-10 keV) to bombard samples
samples are dissolved in a low-volatile liquid –
matrix (e.g. glycerol)
the matrix protects the sample from excessive
radiation damage
similar method: Liquid Secondary Ionization
Mass Spectrometry (LSIMS)

uses more energetic Cs ions (10-30 keV)
Fast atom bombardment ionization
method (FAB) – cont.

both method generate positive and negative ions:


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

FAB can be used in high-resolution mode
primary use: for large, non-volatile molecules for
determining molecular weight
useful structural information about compounds
containing "building blocks":



cations by attachment of proton ([M+1]+) or metal ion ([M+23,
Na]+)
anions by deprotonation ([M-1]-)
polysaccharides
peptides
fragmentation usually occurs at the glycosidic and
peptide bonds
Fast atom bombardment ionization
method (FAB) – cont.
the upper mass limit: 10-20 kDa
 can be used with most types of mass
analyzers
 drawback: a high level of matrix
generated ions

limits sensitivity
 obscure important fragment ions

Plasma desorption ionization

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
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
a highly specialized technique
almost exclusively used with a time of flight
mass analyzer (TOF-MS)
the sample is ionized by bombarding it with the
fission products from Californium 252 (252Cf)
energy: 80-100 MeV
each atom of 252Cf by fission gives 2 particles
moving in opposite directions:


one hits a triggering detector and signals a start
time
the other strikes the sample matrix generating ions
which enter into the TOF-MS analyzer
Plasma desorption ionization (cont.)





the sample ions are most often released as
singly, doubly, or triply protonated moieties
these ions are of fairly low energy so that
structurally useful fragmentation is rarely
observed
for polysaccharides and polypeptides
sequencing information is not available.
the mass accuracy of the method is limited by
the TOF-MS
the technique is useful on compounds with
molecular weights up to at least 45 kDa
Laser desorption ionization



a pulsed laser beam is used to ionize samples
it must be used with either a TOF or a Fourier
transform mass spectrometer
the most widespread used laser types:

a CO2 laser


a frequency-quadrupled neodymium /
yttriumaluminum-garnet (Nd/YAG) laser


emits radiation in the far infrared region
emits radiation in the UV region at 266 nm
without matrix assistance the method is limited
to low molecular weight molecules (<2 kDa)
Laser desorption ionization (cont.)


The power of the method is greatly
enhanced by using matrix assistance
(matrix assisted laser desorption ionization,
or MALDI)
Two matrix materials have widespread use:






N
nicotinic acid and
sinapinic acid
have absorption bands coinciding with the
laser employed
sample molecular weights of up to 200-300
kDa have been successfully analyzed

COOH
a few picomoles of sample are
mixed with the matrix compound
the sample is pulse irradiated
sample ions are ejected from the
matrix into the MS
nicotinic acid
OMe
HO
MeO
COOH
sinapinic acid
Laser desorption ionization (cont.)



The ions have little excess energy and show
little tendency to fragment
useful for mixtures
The mass accuracy:



low when used with a TOF-MS
very high resolution can be obtained with a FT-MS
drawbacks:


background interferences from matrix
assignment of a molecular ion of an unknown
compound can be uncertain
Evaporative ionization methods.
Thermospray MS
a solution of the sample is introduced
into the mass spectrometer by means of
a heated capillary tube
 the tube nebulizes and partially
vaporizes the solvent, forming a stream
of fine droplets
 the droplets enter the ion source
 when the solvent completely evaporates,
the sample ions can be mass analyzed

Electrospray MS




the ion source is operated at or near the atmospheric pressure
it is also called atmospheric pressure ionization or API
the sample solution (usually a polar, volatile solvent) enters the ion
source through a stainless steel capillary, which is surrounded by a
coaxial flow of nitrogen (the nebulizing gas)
The tip of the capillary is maintained at a high potential with respect to
a counter-electrode





the field gradient is up to 5 kV/cm
at the exit of capillary an aerosol of charged droplets is formed
the flow of nebulizing gas directs the effluent toward the MS
droplets in the aerosol shrink as the solvent evaporates - the charged
sample ions are concentrated
a "Coulombic explosion" occurs - the sample ions are released into
the vapor phase
Electrospray MS (cont.)
Electrospray MS (cont.)

multiply charged ions can often be formed


MS records m/z ratios!
the charge of the ion must be known
for the two identified peaks differing by a single
charge:
m1=[Ms+ (z+1)H]/(z+1)
m2=(Ms+ zH)/z
m1, m2 – m/z ratios for the two peaks
Ms – the mass of the sample molecule
H – the mass of a proton
z – the charge
z = (m1 – H)/(m2 – m1 )
 The mass is calculated by the computer software

Electrospray MS (cont.)

growing use in modern MS
very successful ionization by a fairly simple
method
 increasing interest in proteins and smaller
peptides – the best analyzed by ES

Mass analyzers
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Separates the mixture of ions (formed in the
previous step – ionization) by m/z values
Magnetic sector MS (single focusing MS)
Double focusing MS
Quadrupole MS
Ion trap MS
TOF MS
FT MS
Tandem MS
Kinetic energy of ions: E = zV = mv2/2
V- potential of the accelerating electric field
mv
B0 zv =
r
2
mv
r=
zB0
2 2
B r
m/ z=
2V
Magnetic sector – single focusing
MS (cont.)
r is fixed
 B0 is scanned to bring the ions into focus

magnetic sector MS separates ions on
the bases of momentum (mv)
 the ions of the same mass, but different
energies will come into focus at different
points

Double focusing MS



contains electrostatic analyzer (ESA) placed
after the magnetic sector
function: to reduce the energy distribution of
an ion beam by forcing ions of the same
charge and kinetic energy (regardless mass)
to follow the same path
magnetic and electric fields have dispersive
effects on direction and velocity – therefore the
name double focusing

resolution: up to 100,000
allows measurement of "exact masses"
 sensitivity is sacrificed

Quadrupole MS

in comparison with magnetic sector MS:






smaller
cheaper
setup: 4 cylindrical (or of hyperbolic cross-section)
rods, mounted parallel to each other
a constant DC (direct current) voltage modified by RF
voltage is applied to the rods
Ions are introduced to the tunnel formed by the four
rods and travel down the axis
a combination of the two voltages applied at the
appropriate frequency make only ions with a certain
m/z to have a stable trajectory and to pass all the way
to the end of the quadrupole detector
a kind of tunable mass filter
 filtering can be carried out at a very fast
rate (less than 1 s for the entire mass
range)

Quadrupole MS (cont.)

generally inferior to the magnetic sector


better sensitivity than magnetic sector


upper mass range is less than 5000 m/z
most efficient on ions of low velocity (their
ion sources can work at low voltage)
ideal for interfacing to LC systems (low
energy ions), API and electrospray
Ion trap MS

sometimes considered as a variant of the
quadrupole



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

the appearance and operation of the two are
related
more versatile
has greater potential for development
it can "trap" ions for relatively long periods of
time
sequential ejection of ions into a detector
produces a conventional mass spectrum

three electrodes:



one ring electrode with a hyperbolic inner surface
(sin RF voltage)
two hyperbolic endcap electrodes (ground, DC or
AC voltage)
controlling the three parameters (voltages) a
wide variety of experiments can be run
Ion trap MS (cont.)

three basic modes of operation:
1.
RF voltage fixed, no DC bias between the
endcap and ring electrodes


1.
all ions above a certain m/z ratio are trapped
by increasing RF, ions are sequentially ejected and
detected
DC potential across the endcaps (mostly used
mode)


both a low and high-end cut-off (m/z) ions
if needed, only one ion mass can be selected
(selective ion monitoring)
Ion trap MS (cont.)
3.
similar to the second, with the addition of an
auxiliary oscillatory field between endcap
electrodes




kinetic energy is added selectively to a particular ion
slow increasing of ion Ek leads to a fragmenting
collision
almost 100% of MS-MS efficiency
undesired ions (solvent or matrix ions in FAB or
LSIMS experiments) can be rejected
Time-of-flight MS

the concept:



ions are accelerated through a potential (V) and are
then allowed to "drift" down a tube to a detector
assumption: all ions arriving at the beginning of the
drift tube have the same energy: zeV=mv2/2
ions of different mass will have different velocities:
2zeV
v=
m

if L is the length if the drift tube, time of flight is:
L2 m
t=
2zeV
Time-of-flight MS (cont.)

TOF spectrometers must use pulsed ionization
techniques like:






plasma desorption ionization
MALDI (matrix assisted laser desorption ionization)
resolution: usually less than 20,000
the mass range is unlimited
excellent sensitivity (no resolving slits)
most useful for large biomolecules
Fourier transform MS




ions are held in a cell with an electric trapping potential
within a strong magnetic field
within the cell, each ion orbits in a direction perpendicular
to the magnetic field, with a frequency proportional to the
ion's m/z
an RF pulse applied to the cell brings all of the cycloidal
frequencies into resonance simultaneously to yield an
interferogram (similar to the free induction decay (FID)
signal in NMR or the interferogram generated in FTIR
instruments)
the interferogram (which is a time domain spectrum) is
Fourier transformed into a frequency domain spectrum the conventional m/z spectrum
Fourier transform MS (cont.)





extremely high field superconducting magnets
are used
very high mass detection is possible
high sensitivity
high resolution
convenient for coupling to chromatographic
instruments and various ionization methods

also very good for analyzing small molecules
Tandem MS (MS-MS)



from the initial fragmentation a "parent" ion is selected
it is induced to fragment further giving rise to
"daughter" ions
In complex mixtures daughter ions provide
unequivocal evidence for the presence of a known
compound


for unknown or new compounds, the daughter ions provide
potential for further structural information
for mixtures, tedious separation and isolation of pure
compounds prior MS analysis can be avoided
(possibility to extract a single ion and monitor its
fragmentation)
Interpretation of EI mass spectra

in this course: interpretation of mass spectra
obtained by EI MS



a lot of structural information
very useful in organic chemistry for structure
elucidation
what happens when a 70 eV electron beam
hits the molecule of interest?

the simplest event – removing of an electron from
the molecule → molecular ion (radical cation)
CH3OH + e- → CH3OH+˙ + 2e-
Mass spectrum
if the charge can be localized on one particular
atom:
CH3OH → positively charged fragment + radical
 Examples of possible cleavages for methanol:

Mass spectrum (cont.)




If some of the molecular ions remain intact long enough
to reach the detector, we see a molecular ion peak
important for determination of the molecular weight of the
compound
a mass spectrum: a presentation of the masses of the
positively charged fragments (including the molecular ion)
versus their relative concentrations
The most intense peak in the spectrum is called the base
peak



it is assigned a value of l00%
intensities of other peaks (including the molecular ion
peak) are given as percentages of the base peak
the molecular ion peak may be sometimes the base peak
Recognition of the molecular ion peak

difficulties for recognition of the molecular ion:



weak peak
it doesn't appear at all
the "nitrogen rule":

a molecule of even-numbered molecular weight
must contain either:



no nitrogen or
an even number of nitrogen atoms
an odd-numbered molecular weight indicates an
odd number of nitrogen atoms
Recognition of the molecular ion
peak (cont.)

The intensity of the molecular ion peak
depends on the stability of the molecular ion


the most stable molecular ions are those of purely
aromatic systems.
If substituents that have favorable modes of
cleavage are present:


the molecular ion peak will be less intense
the fragment peaks will be relatively more intense in
general.
Recognition of the molecular ion
peak (cont.)

group of compounds which usually give
prominent molecular ion peaks:
R' S
aromatic compds

O
cyclic compds
R
CH3CH2CH3
organic sulfides short, n-alkanes
SH
mercaptans
group of compounds which usually give
recognizable molecular ions:
R" RNH2
ketones amines
R' C
conjugated alkenes
R"
O
R' C OR"
esters
O
R' O R" R C OH
ethers
carboxylic
acids
O
O
H R C NH2
amides
aldehydes
R C
R X
halides
Recognition of the molecular ion
peak (cont.)

rarely detectable molecular ions:
aliphatic alcohols
 nitrites
 nitrates
 nitro compounds
 nitriles
 highly branched compounds

Recognition of the molecular ion
peak (cont.)

indications of molecular peaks:
M-15 peak (loss of CH3)
 M-18 peak (-H2O)
 M-31 peak (-OCH3)
 M-1 peak is common
 M-2 peak – occasionally seen – loss of H2
 M-3 peak – very rare, but possible from alcohols
 M-3 to M-14: either contaminants or M is not the
molecular ion, but a fragment ion peak
 M-19 to M-25 unlikely, except for F-compds (-F=19,
-HF=20)
 M-16 to M-18 – only if an oxygen atom is present
(-O=16, -OH=17, -H2O=18)

Determination of a molecular
formula

So far, the mass spectrum in terms of unit resolutions

the unit mass of the molecular ion of C7H7NO is mlz 121- (the
sum of the unit masses of the most abundant isotopes
(7 X 12) + (7 X 1) + (1 X 14)+ (1 X 16) = 121

there are molecular species which contain the less
abundant isotopes




peaks at M+1, M+2, etc.
isotopes contributing to the M+1 peak: 13C, 2H, 15N, 17O
the only contributor to the M+2 peak: 18O (very low
abundance, M+2 can not be detected)
If only C, H, N, O, F, P and I are present in the
molecule CnHmNxOy (F, P and I are monoisotopic):
%(M+1) ≈ 1.1n + 0.36x
%(M+2) ≈ (1.1n)2/200 + 0.2y
Determination of a molecular
formula (cont.)
If these isotope peaks are intense
enough to be measured accurately, the
above calculations may be useful in
determining the molecular formula
 contribution to the M+2 peak:

one S-atom in a molecule: 4.40% (34S)
 one Si-atom in a molecule: 3.35% ( 30Si)

High-resolution molecular ion



A molecular formula (or fragment formula) can
often be derived from a sufficiently accurate
mass measurement alone (high-resolution
mass spectrometry).
example: four compds CO, N2, CH2N and C2H4
have equal unit masses of 28
their exact masses:




CO: 12.0000 + 15.9949 = 27.9949
N2: 2 x 14.0031 = 28.0062
CH2N: 12.0000 + 2 x 1.0078 + 14.0031 = 28.0187
C2H4: 28.0312
Index of hydrogen deficiency

very important information from MS: molecular
formula


kind and numbers of atoms in a molecule
the index of hydrogen deficiency



the number of pairs of hydrogen atoms that must be
removed from the corresponding "saturated" formula to
produce the molecular formula of the compound of interest
synonym: the degree of unsaturation
the degree of unsaturation: the sum of the number
of rings, the number of double bonds and twice the
number of triple bonds
Index of hydrogen deficiency (cont.)

For a compound of molecular formula
CnHmXxNyOz the index can be calculated:
Index = n – m/2 – x/2 + y/2 + 1
 example: C7H7NO: index = 5
O
O
H2N
NH2

for benzene: index = 4
H
Index of hydrogen deficiency (cont.)

for compounds containing an atom in a
higher valence state (like S or P) polar
structures must be used (the Lewis octet
rules must be obeyed)
Index of hydrogen deficiency (cont.)



The formula above for the index can be
applied to fragment ions as well
When applied to even-electron (all electrons
paired) ions, the result is always an odd
multiple of 0.5.
example: consider C7H5O+


the index is 5.5
the possible structure:
C O
Fragmentation

representations of the radical cation:
A: delocalized structure
 B and C: unpaired electron and positive
charge are localized

Fragmentation (cont.)


fragmentation of the molecular ion:

homolytic cleavage

heterolytic cleavage
Simultaneous or consecutive cleavage of
several bonds may occur as well
Fragmentation (cont.)

The probability of cleavage of a particular
bond is related to:




the bond strength
the possibility of low energy transitions
the stability of the fragments
mass spectrometry is concerned with the
consequences suffered by an organic
molecule at a vapor pressure of about 10 -6 mm
Hg struck by an ionizing electron beam
General rules for predicting
prominent peaks in EI spectra
The relative height of the molecular ion
peak is greatest for the straight-chain
compound and decreases as the
degree of branching increases
2. The relative height of the molecular ion
peak usually decreases with increasing
molecular weight in a homologous
series
1.

Fatty esters appear to be an exception.
General rules for predicting
prominent peaks (cont.)
3.
Cleavage is favored at alkyl-substituted
carbon atoms: the more substituted, the
more likely is cleavage

4.
stability of a carbocation
CH3+ < R-CH2+ < R2CH+ < R3C+
Double bonds favor allylic cleavage and give
the resonance-stabilized allylic carbocation

This rule does not hold for simple alkenes (ready
migration of the double bond), but it does hold for
cycloalkenes
5.
Double bonds, cyclic structures, and
especially aromatic (or heteroaromatic) rings
stabilize the molecular ion and thus increase
the probability of its appearance
General rules for predicting
prominent peaks (cont.)
6.
Saturated rings tend to
lose alkyl side chains at
the α bond


This is merely a special
case of branching. The
positive charge tends to
stay with the ring
fragment
unsaturated rings can
undergo a retro-DielsAlder reaction
General rules for predicting
prominent peaks (cont.)
7.
In alkyl-substituted
aromatic compds
cleavage is very
probable at the
bond β to the ring
giving the
resonancestabilized benzyl
ion or, more likely,
the tropylium ion
General rules for predicting
prominent peaks (cont.)
8.
The C-C bonds next to a heteroatom are
frequently cleaved, leaving the charge on the
fragment containing the heteroatom

8.
its nonbonding electrons provide resonance
stabilization
Cleavage is often associated with elimination
of small, stable, neutral molecules (carbon
monoxide, olefins, water, ammonia,
hydrogen sulfide, hydrogen cyanide,
mercaptans, ketene, or alcohols) often with
rearrangement
General rules for predicting
prominent peaks (cont.)
These fragmentation rules apply to EI
mass spectrometry
 molecular ions obtained by other ionizing
techniques (CI, etc.):

have much lower energy
 very different fragmentation patterns
 different rules govern their fragmentation

Rearrangements

Rearrangements ions – fragments
formed by intramolecular atomic
rearrangement during fragmentation


not result of simple cleavage of bonds
very common rearrangements: those
involving migration of hydrogen atoms in
molecules that contain heteroatom
McLafferty rearrangement


an important example of rearrangement
conditions for McLafferty rearrangement:




an appropriately located heteroatom in a molecule
(e.g. O, N...)
a π system (usually double bond)
an abstractable hydrogen atom in γ position to the
C=O system
rearrangement is followed by elimination of a
stable, neutral molecule (in this case the
alkene product)
McLafferty rearrangement (cont.)
Rearrangements (cont.)


how to recognize rearrangement peaks?
consider m/z number for fragment ions and for their
corresponding molecular ions:

cleavage



an even-numbered molecule ion gives an odd-numbered
fragment
an odd-numbered molecular ion gives an even-numbered
fragment
rearrangement

observation of a fragment ion mass different by 1 unit from that
expected for a fragment resulting from simple cleavage
Mass spectra of saturated
hydrocarbons




a lot of MS studies related to the petroleum
industry
general rules about fragmentation apply
rearrangement peaks are not usually intense
(random rearrangements)
The molecular ion peak of a straight-chain,
saturated hydrocarbon is always present


low intensity for long-chain compounds
The fragmentation pattern is characterized by
clusters of peaks, and the corresponding
peaks of each cluster are 14 mass units (CH2)
apart
Mass spectra of saturated
hydrocarbons (cont.)

The largest peak in each cluster represents a CnH2n+1
fragment






it occurs at mlz = 14n + 1
this is accompanied by CnH2n and CnH2n-1 fragments
The most abundant fragments are at C3 and C4
the fragment abundances decrease in a smooth curve
down to [M - C2H5]+
the [M - CH3]+ peak is usually very weak or missing
hydrocarbons from C8 and above have very similar MS
spectra

identification: molecular ion peak
Mass spectra of saturated
hydrocarbons (cont.)
Spectra of branched saturated
hydrocarbons are very similar to those of
straight-chain compounds
 the smooth curve of decreasing
intensities is broken by preferred
fragmentation at each branch

Mass spectra of saturated
hydrocarbons (cont.)
Mass spectra of cyclic, saturated
hydrocarbons

A saturated ring increases the relative intensity
of the molecular ion peak


Fragmentation of the ring:



it favors cleavage at the bond connecting the ring to
the rest of the molecule
usually characterized by loss of two carbon atoms
(C2H4 (28) and C2H5 (29))
a greater proportion of even-numbered mass ions
in MS than in the spectrum of an acyclic
hydrocarbon
The characteristic peaks are in the CnH2n-1 and
CnH2n-2 series
Example: MS of cyclohexane

molecular ion is more intense than in
acyclic compounds (fragmentation
requires cleavage of two bonds)
MS of alkenes


alkenes (especially polyalkenes) have a
distinct peak in MS
difficult to determine location of the double
bond


can easily migrate in the fragment
in cyclic and polycyclic alkenes it's easier to
determine the double bond position


a strong tendency for allylic cleavage – the position
of the double bond is fixed
also in the case of conjugation with carbonyl group
MS of alkenes (cont.)
acyclic alkenes are characterized by
clusters of peaks at intervals of 14 units
 the CnH2n-1 and CnH2n peaks are more
intense than the CnH2n+1 peaks


example: the MS spectrum of β-myrcene

m/z 41 – C3H5+ (CnH2n-1, n=3)

m/z 55 - C4H7+ (n=4)

m/z 69 - C5H9+ (n=5)
MS of β-myrcene

m/z 41: involved in isomerization:
MS of β-myrcene (cont.)

m/z 67 and 69 ions are formed by
cleavage of a bi-allylic bond
Cyclic alkenes
usually show a distinct
molecular ion peak
 A unique mode of
cleavage is the retroDiels-Alder reaction
(example of limonene)

two isoprene molecules
 rearrangement occurs:
one fragment is a
neutral molecule

Aromatic and aralkyl hydrocarbons
An aromatic ring in a molecule stabilizes
the molecular ion peak
 in the case of naphthalene:


the molecular ion peak is the base peak
Aromatic and aralkyl hydrocarbons
(cont.)




An alkyl-substituted benzene ring frequently
gives a prominent peak (often the base peak)
at mlz 91 – tropylium ion
Branching at the α-carbon leads to masses
higher than 91, by increments of 14
the largest substituent is eliminated most
readily
branching at the α-carbon does not reduce
intensity of the m/z 91 ion

the highly stabilized fragment can be formed by
rearrangement
Tropylium or benzylic cation?

tropylium ion seems to be more stable

xylenes readily lose a methyl group, but
toluene does not
More than two C-atoms in side
chain

Hydrogen migration with elimination of a
neutral alkene molecule accounts for the
peak at m/z 92

it is an example of a rearrangement reaction
Monoalkylbenzenes

A characteristic cluster of ions resulting
from a cleavage and hydrogen migration
in monoalkylbenzenes appears at m/z 77
(C6H5+), 78 (C6H6+), and 79 (C6H7+)
Hydroxy compounds. Alcohols

The molecular ion peak :




of a primary or secondary alcohols is usually quite
small
for a tertiary alcohols is often undetectable
for obtaining information about molecular weight it
is necessary to use other ionization methods (e.g.
CI)
Typical: cleavage of the C-C bond next to the
oxygen atom


1o alcohols – peak at m/z 31 (+CH2OH)
2o and 3o alcohols – analogous peaks +CHROH and
+
CRR'OH
Alcohols (cont.)
again: the largest substituent is expelled
most readily
 less favored fragmentation: cleavage of
C-H bond next to the oxygen atom (M-1)
 after cleavage of oxygen – typical
fragmentation pattern as in
hydrocarbons (in long-chain alcohols of
6 or more C-atoms)


series of peaks of decreasing intensities
Alcohols (cont.)

sometimes in spectra of primary alcohols


ion at M - 18 (loss of water)
together with water, an alkene molecule can
be eliminated from the primary alcohols
Alcohols (cont.)

a good indication for primary alcohols:


a peak at m/z 31 (+CH2OH), if it is more
intense than peaks at m/z 45, 59, 73...
have in mind:

the first-formed ion of a secondary alcohol
can decompose further to give a moderately
intense mlz 31 ion.
Cyclic alcohols
they fragment by complicated pathways
 example: cyclohexanol (M = m/z 100)
gives fragments:


C6H10O+ by loss of the α-hydrogen

C6H10+ loss of H2O

C3H5O+ (m/z 57) by a complex ring cleavage
pathway
Benzyl alcohol and homologs




generally, the parent
peak is strong
a moderate benzylic
peak (M-OH) may be
present (cleavage of βbonds to the ring)
M-1, M-2 and M-3 peaks,
by a complex sequence
Benzyl alcohol itself:



M-1 ion
C6H7+ (loss of CO)
C6H6+ (loss of H2)
Benzyl alcohol and homologs (cont.)

Loss of H2O to give a distinct M - 18 peak – a
common feature, especially for some orthosubstituted benzyl alcohols

The aromatic cluster at m/z 77, 78, and 79
resulting from complex degradation is
prominent here also
Phenols


A conspicuous molecular ion peak facilitates
identification of phenols
for phenol itself:



molecular ion is the base peak
M-1 peak is small
in cresols:

the M-1 peak is larger than the molecular ion


easy benzylic C-H cleavage
other usual peaks in MS of phenols:



rearrangement peak at m/z 77
M-28 (loss of CO)
M-29 (loss of CHO)
MS of o-ethylphenol
Ethers. Aliphatic ethers
The molecular ion peak is small
 intensity of the M+ or the [M+1]+ can be
enhanced by a larger sample size



indication of the O-atom presence:


(H˙ transfer during ion-molecule collision)
strong peaks at m/z 31, 45, 59, 73...(RO+
and ROCH2+ fragments)
two principal ways of fragmentation
Aliphatic ethers (cont.)
1.
Cleavage of the C-C bond next to the oxygen
atom (α,β-bond)

this can give rise to the base peak
Aliphatic ethers (cont.)
C-O bond cleavage with the charge
remaining on the alkyl fragment
 long-chain ethers have spectra
dominated by the hydrocarbon pattern
2.
Acetals and ketals


acetals – a special class of ethers
acetals' MS:






extremely weak M+ ion
prominent peaks at M-R and M-OR (or
M-OR')
weak peak at M-H
as usual – elimination of the largest
group is preferred
similarly to ethers, the first-formed
oxygen-containing fragments can
decompose further with hydrogen
migration and alkene elimination
Ketals behave similarly
Aromatic ethers



The molecular ion peak is prominent
Primary cleavage occurs at the bond β to the
ring
the first-formed ion can decompose further
Aromatic ethers (cont.)




When the alkyl portion of an
aromatic alkyl ether is C2 or
larger, cleavage β to the ring is
accompanied by hydrogen
migration (as for alkyl benzenes)
cleavage is mediated by the ring
rather than by the oxygen atom
C-C cleavage next to the oxygen
atom is insignificant
diphenyl ethers show peaks at
M-H, M-CO and M-CHO by
complex rearrangements
Aliphatic ketones


the molecular ion peak is usually quite
pronounced
major fragmentation peaks result from
cleavage at one of the C-C bonds adjacent to
the oxygen atom

the charge remains with the resonance stabilized
acylium ion
Aliphatic ketones (cont.)

cleavage is mediated by the oxygen atom (as
with alcohols and ethers)



peaks at m/z 43, 57 or 71...
the base peak – usually results of a loss of the
larger alkyl group
When one of the alkyl chains attached to the
C=O group is C3 or longer, cleavage of the C-C
bond once removed (α,β-bond) from the C=O
group occurs with hydrogen migration to give a
major peak (McLafferty rearrangement)
Aliphatic ketones (cont.)
Aliphatic ketones (cont.)

in long-chain ketones - the hydrocarbon
peaks are indistinguishable from the acyl
peaks (without the aid of high-resolution
techniques)


the mass of the C=O unit (28) is the same
as two methylene units
the multiple cleavage modes in ketones
→ difficulties in the determination of the
carbon chain configuration
Cyclic ketones




The molecular ion peak is prominent
As with acyclic ketones, the primary cleavage of
cyclic ketones is adjacent to the C=O group
however, the ion thus formed must undergo
further cleavage in order to produce a fragment
The base peak in the MS of cyclopentanone and
of cyclohexanone is at m/z 55
Cyclic ketones (cont.)
Aromatic ketones


The molecular ion peak is prominent
Cleavage of aryl alkyl ketones occurs at the bond β to
the ring




a characteristic ArC≡O+ fragment (when Ar=Ph, m/z 105)
usually the base peak
loss of CO from this fragment gives the "aryl" ion (m/z
77 in the case of acetophenone).
if alkyl chain has 3 or more carbon atoms:

the same fragmentation pattern as for aliphatic ketones



cleavage of the α,β C-C bond through cyclic transition state
migration of H-atom
elimination of an alkene and forming a stable ion
Aromatic ketones (cont.)
Aliphatic aldehydes


The molecular ion peak is usually visible
Cleavage of:




the C-H bond → M-1 peak
the C-C bonds next to the oxygen atom → M-R
(peak m/z 29, CHO+).
M-1 peak - a good diagnostic peak, even for
long-chain aldehydes
the m/z 29 peak is not as characteristic

can originate from C2H5+ ion in higher aldehydes
(>C4)
Aliphatic aldehydes

In the C4 and higher aldehydes:
a major peak at m/z 44, 58 or 72..., (depending
on the α-substituents)
 a result of McLafferty rearrangement: the α,β CC bond cleavage, with hydrogen migration and
cyclic transition state

Aliphatic aldehydes (cont.)

in normal-chain aldehydes – diagnostic peaks are:




M-18 (loss of water)
M-28 (loss of ethylene)
M-43 (loss of CH2=CHO˙)
M-44 (loss of CH2=CHOH)
Aromatic aldehydes

characterized by:
large molecular ion peak
 M-1 peak (Ar-C≡O+); always large (in some
cases larger than M+ peak)
 the Ph-C≡O+ ion can eliminate CO, giving
phenyl ion (m/z 77)
 phenyl ion eliminates acetylene and gives
C4H3+ ion, m/z 51

Aliphatic carboxylic acids


the molecular ion peak of a
normal chain monocarboxylic
acids is weak, but noticable.
the most characteristic
(sometimes base peak) is m/z
60


McLafferty rearrangement
Branching at the α-carbon
enhances this cleavage
Aliphatic carboxylic acids (cont.)

prominent peaks in short-chain acids:
M – OH
 M – COOH
O

R
C
OH

spectra of long-chain acids resemble the
series of hydrocarbon clusters
MS spectrum of decanoic acid
Aromatic carboxylic acids
The molecular ion peak is intense
 other prominent peaks:

M – 17 (OH)
 M – 45 (COOH)
 loss of H2O (M – 18) is prominent if a
hydrogen-bearing ortho group is available

Aliphatic esters

The molecular ion peak of a methyl ester of a
straight-chain aliphatic acid is usually distinct


The molecular ion peak is weak in the range
m/z 130 to ~200


even waxes usually show a discernible molecular
ion peak.
becomes somewhat more intense beyond this
range
the most characteristic peak results from:


the McLafferty rearrangement
the cleavage of α,β bond with regard to C=O group
Aliphatic esters
(cont.)



McLafferty rearrangement
of aliphatic esters
the cleavage of α,β−bond
with regard to C=O group
Me-ester of an aliphatic
acid, unbranched at the
α-C atom, gives a strong
peak at m/z 41 (often the
base peak in C6-C26
methyl esters
O
OCH3
Aliphatic esters (cont.)

Possible fragmentations:
R+ ion prominent in short-chain
esters, but becomes almost
undetectable from Me-hexanoate
on
 R-C≡O+ - typical ion for esters (for
Me-esters it appears at M-31);



the base peak for MeAc
[OR']+ and [COOR']+ ions are of
little importance
Aliphatic esters (cont.)

if the acid portion (straight-chain) of the
ester molecule is predominant:

the fragmentation pattern is the same as for
free acids
C-C cleavage: alkyl ion (m/z 29, 43, 57,...) and
oxygen-containing ion CnH2n-1O2+ (m/z 59,
73,87,...)
 hydrocarbon clusters at intervals of 14 mass
units

MS of methyl-octanoate
Esters of long-chain alcohols

diagnostic peak at m/z 61, 75, or 89...
elimination of the alkyl moiety in the form of
alkene
 transfer of two hydrogen atoms to the
fragment containing the oxygen atoms


esters of dibasic acids
ROOC(CH2)nCOOR, in general, give
recognizable molecular ion peaks
Benzyl and phenyl esters

Benzyl acetate (also furfuryl acetate and other
similar acetates) and phenyl acetate eliminate
the neutral molecule ketene


frequently this gives rise to the base peak
prominent peaks for benzyl acetate:


CH3C=O+ at m/z 43
[C7H7]+ , m/z 91
Esters of aromatic acids

Me-esters (ArCOOMe) have prominent
molecular ion peak




its intensity decreases with increasing number of Catoms in alcohol moiety
the base peak: elimination of RO·
another prominent peak: elimination of RCOO·
increasing alkyl moiety leads to three typical
modes of cleavage:



McLafferty rearrangement
rearrangement of two hydrogen atoms and
elimination of an allylic radical
retention of the positive charge by the alkyl group
Esters of aromatic acids (cont.)




McLafferty rearrangement gives rise to a peak
[ArCOOH]+
rearrangement of two hydrogen atoms gives
the protonated aromatic acid [ArCOOH2]+
The third mode of cleavage gives the alkyl
cation R+
ortho-substituted benzoates eliminate ROH
through the general "ortho" effect (described
for aromatic acids)

e.g. the base peak of Me-salicylate iz m/z 120; by
elimination of CO gives a strong peak at m/z 92
Phthalates
all esters of phthalic acid have a
characteristic peak at m/z 149
 originates from higher esters (long-chain
esters)

often used as plasticizers
 strong 149 peak may indicate contamination
 it is probably formed by two ester cleavages
involving the shift of two hydrogen atoms
and elimination of H2O

Lactones

The molecular ion peak of five-member ring lactones
is distinct


It is weaker when an alkyl substituent is present at C4
The base peak (mlz 56) of γ-valerolacton and the
same strong peak of butyrolacton are due to
fragmentation and loss of acetaldehyde
Aliphatic amines

the molecular ion peak of an aliphatic
monoamine is an odd number



usually very weak
undetectable in long-chain or highly branched amines
The base peak frequently results from C-C
cleavage next to the nitrogen atom

for primary amines, unbranched at the α-carbon, this is
m/z 30 (CH2NH2+)
Aliphatic amines (cont.)

when there is branching at the α-carbon


loss of the largest branch is preferred
primary straight-chain amines show a
homologous series of peaks of progressively
decreasing intensity




the cleavage og the ε-bond is slightly more important
than at the neighboring bonds
ions at m/z 30, 44, 58,... resulting from cleavage at
C-C bonds successively removed from the nitrogen
atom
retention of the charge on the N-containing fragment
noticeable hydrocarbon fragmentation pattern as well
Aliphatic amines (cont.)

Cyclic fragments occur during fragmentation of
longer chain amines


commonly formed 6- and 5-membered rings
fragmention of straight-chain primary amines
is similar to those in aliphatic alcohols

m/z 30, 44, 58, 72...
Amino acid esters

cleavage occurs at both C-C bonds next
to the N atom
loss of carbalkoxy group (-COOR') is
preferred
 the aliphatic amine fragment
decomposes further to give a peak at
m/z 30

Cyclic amines


the molecular ion peaks are usually intense,
unless there is substitution at the α position
Primary cleavage at the bonds next to the N
atom leads either to:
 loss of an α-hydrogen atom to give a strong M-1


peak, or
opening of the ring;
the latter is followed by elimination of ethylene
and gives ions


m/z 43 (·CH2―+NH=CH2)
m/z 42 (CH2=N+=CH2)
Aromatic amines (anilines)
the odd numbered molecular ion peak is
intense
 loss of one of the amino H atoms of
aniline gives a moderately intense M-1
peak
 Ioss of a neutral molecule of HCN
followed by loss of a hydrogen atom
gives prominent peaks at m/z 66 and 65
respectively

Anilines (cont.)

in alkyl aryl amines, cleavage of the C-C
bond next to the nitrogen atom is
dominant

the heteroatom controls cleavage, like in
alkyl aryl ethers
Aliphatic amides


the molecular ion peak of straight-chain
monoamides is usually noticeable
the dominant modes of cleavage depend on:



the length of the acyl moiety and
the lengths and number of the alkyl groups
attached to the nitrogen atom
the base peak in all straight-chain primary
amides higher than propionamide results from
McLafferty rearrangement (m/z 59,
H2NC(=OH+)CH2˙)
Aliphatic amides (cont.)

primary amides give a strong peak at
m/z 44 from cleavage of the R-CONH2
bond (O=C=+NH2)
the base peak in C1-C3 primary amides and
in isobutyramide
 γ,δ C-C cleavage gives a moderate peak at
m/z 86, possibly accompanied by cyclization

Aliphatic amides (cont.)


secondary and tertiary amides with an
available hydrogen on the γ-carbon of the acyl
moiety and methyl groups on the N atom show
the dominant peak resulting from the
McLafferty rearrangement
when the N-alkyl groups are C2 or longer and
the acyl moiety is shorter than C3, another
mode of cleavage predominates


cleavage of the N-alkyl group β to the nitrogen atom
cleavage of the carbonyl C-N bond and migration of
an α-hydrogen atom of the acyl moiety (eliminating
a neutral ketene molecule)
Aromatic amides

loss of NH2 → stable benzoyl cation →
phenyl cation
Aliphatic nitriles
the molecular ion peaks are weak or
absent (except for acetonitrile and
propionitrile)
 the M+1 peak can usually be located by
increasing the sample size
 a weak, but diagnostically useful M-1
peak is formed by loss of an α-hydrogen:
RCH=C=N+

Aliphatic nitriles (cont.)

the base peak of straight-chain C4-C9
nitriles is at m/z 41


results from hydrogen rearrangement in a
six-membered transition state, similar to a
McLafferty rearrangement, giving the ion
CH2=C=N+-H
the peak is of low diagnostic value (ion
at m/z 41 give all molecules with
hydrocarbon chains – C3H5+)
Aliphatic nitriles (cont.)

in straight-chain nitriles from C8 and
higher:
a characteristic and intense peak at m/z 97
 sometimes the base peak

Aliphatic nitriles (cont.)

characteristic series of homologous
peaks of even mass number
m/z 40, 54, 68, 82...
 result of a simple cleavage of C-C bond (not
the one next to the N atom)
 ion type: (CH2)nC≡N+

Aliphatic nitro compounds
the molecular ion peak (odd number) of
an aliphatic mononitro compound is
weak or absent (except in the lower
homologs)
 the main peaks: M - NO2


presence of nitro group:
a peak at m/z 30 (NO+)
 a smaller peak at m/z 46 (NO2+)

Aromatic nitro compounds
the molecular ion peak (odd number for
one N-atom) is strong
 prominent peaks:


elimination of an NO2 radical (M-46)


elimination of a neutral NO molecule with
rearrangement


the base peak in nitrobenzene
result: phenoxy cation (M-30)
both are diagnostic peaks
Aromatic nitro compounds (cont.)

isomeric o-, m- and p-nitroanilines


strong molecular ions (even number)
prominent peaks resulting from:

a loss of an NO2 group (M-46) → m/z 92


further loss of HCN → m/z 65
a loss of NO (M-30) → m/z 108
 follow the loss of CO → m/z 80
all three isomers give
similar MS spectra

o-isomers eliminate OH and
give small peak at m/z 121

Aliphatic nitrites


the molecular ion peak (odd number for one Natom in the molecule) is weak or absent
a large peak at m/z 30 (NO+)


a large peak at m/z 60, CH2=+ONO (if there is
no branching at the α-carbon)



often the base peak
results from cleavage of the C-C bond next to the
ONO group
an α-branch can be identified by a peak at m/z
74, 88, or 102...
diferentiation from nitro compounds:

no large peak at m/z 46
Aliphatic nitrates
the molecular ion peak (odd number for
one N-atom in the molecule) is weak or
absent
 a prominent (often the base) peak:



by cleavage of the C-C bond next to the
ONO2 group and loss of the heaviest alkyl
group attached to the α-carbon
also prominent peak at m/z 46 (NO2+)
Halogen compounds

a compound that contains one chlorine atom
will have an M+2 peak approximately 1/3 the
intensity of the molecular ion


the presense of a molecular ion containing the 37Cl
isotope (rel.abundance 24.5%)
a compound that contains one bromine atom
will have an M+2 peak almost equal in
intensity to the molecular ion

the presense of a molecular ion containing the 81Br
isotope (rel.abundance 49.5%)
Halogen compounds (cont.)

A compound that contains two chlorines, or
two bromines, or one chlorine and one
bromine:



three Cl atoms in a molecule:


a distinct M+4 peak, in addition to the M+2 peak
reason: the presence of a molecular ion containinig
two atoms of the heavy isotope
M+2, M+4 and M+6 peaks
the relative abundances of the peaks
(molecular ion, M+2, M+4, etc.) have been
calculated for compounds containing Br and/or
Cl atoms
Halogen compounds (cont.)
Halogen compounds (cont.)
Sulfur compounds

can be readily recognized by the M+2
peak


4.4%, contribution of the isotope 34S
the number of S-atoms can be
determined by the intensity of the M+2
peak
Aliphatic thiols

The molecular ion peak (except for
higher tertary thiols), is usually strong
enough


the M+2 peak can be accurately measured
the cleavage modes resemble those of
alcohols


cleavage of the C-C bond (α,β-bond) next to
the SH group gives the characteristic ion
CH2=SH+ (m/z 47)
other fragments:



m/z 61 (50% of intensity of 47)
m/z 75, low intensity
m/z 89 relatively intense; stabilized by cyclization:
Aliphatic thiols (cont.)

primary thiols: M-34 (loss of H2S), then
elimination of (CH2=CH2)n


the homologous series of ions
M - H2S - (CH2=CH2)n
secondary and tertiary:

cleavage at the α-C atom and loss of the largest
group:




prominent peaks: M-CH3, M-C2H5,...
peak at m/z 47 – rearrangement peak
M-33 (loss of HS in secondary thiols)
long-chain thiols – superimposed hydrocarbon
pattern
Aliphatic sulfides

the molecular ion – intense enough


the M+2 peak can be accurately measured
the cleavage modes resemble those of ethers



cleavage of one or the other of the C-C bonds (α,βbonds)
loss of the largest group is favored
characteristic ion RH=SH+
Aliphatic sulfides (cont.)

if a sulfide is unbranched at either δ−carbons:



the ion CH2=SH+ (m/z 47) with intensity like in thiols
distinction: missing M - H2S or M – SH ions
In all sulfides except tertiary ones:



moderate to strong peak at m/z 61
if there is an α-methyl substituent, the peak is CH3CH=SH+ (results
from the double cleavage)
in the straight-chain sulfides, m/z 61 peak is 3-membered ring:
Aliphatic sulfides (cont.)

Sulfides give a characteristic ion by cleavage
of the C-S bond


charge retains on sulfur
the resulting RS+ ion gives a peak at m/z


32+CH3, 32+C2H5, 32+C3H7,...
esspecially favored ion at m/z 103

formation of a rearranged cyclic ion
Aliphatic sulfides (cont.)
Aliphatic sulfides (cont.)


as with the long-chain ethers, the hydrocarbon
pattern may dominate the spectrum of longchain sulfides
Alifatic disulfides


for up to 10 C-atoms – strong molecular ion
the major peak: by cleavage of one of the C-S
bonds



charge retains on the alkyl fragment
another intense peak: RSSH fragment
other ions: RS+, RS+-1 and RS+-2
Aliphatic chlorides


The molecular ion peak is detectable only in
the lower monochlorides
Fragmentation of the molecular ion is
mediated by the chlorine atom


however, to a much lesser degree than is the case
in oxygen-, nitrogen-, or sulfur containing
compounds
Cleavage of a straight-chain monochloride at
the C-C bond adjacent to the chlorine atom
gives a small peak at m/z 49: CH2=Cl+ (and
the isotope peak at m/z 51)
Aliphatic chlorides (cont.)

Cleavage of the C-Cl bond



a small Cl- peak and an R+ peak
R+ peak is prominent in the lower chlorides but quite small
when the chain is longer than about C5
Straight-chain chlorides longer than C6 give:



C3H6Cl+, C4H8Cl+ and C5H10Cl+ ions
C4H8Cl+ ion forms the most intense (sometimes the base)
peak
its stability is explained by 5-membered cyclic structure
Aliphatic bromides and iodides

bromides:


the remarks for chlorides generally apply to the
corresponding bromides
iodides:




the strongest molecular ion
no distinctive isotope peak (iodine is monoisotopic
element)
cleavage pattern is like in chlorides and bromides
C4H8I+ ion is not as evident as the corresponding
chlorides and bromides
Aliphatic fluorides


give the weakest molecular ion peak of the
aliphatic halides
Fluorine is monoisotopic, and its detection in
polyfluoro compounds depends on:




the most characteristic is mlz 69 (the CF3+ ion)


suspiciously small isotopic peaks relative to the
molecular ion
the intervals between peaks
characteristic peaks
the base peak in all perfluorocarbons
prominent peaks at m/z 119, 169, 219...

increments of CF2
Aliphatic fluorides (cont.)

the stable ions (give intense peaks):





C3F5+ (m/z 131)
C4F7+ (m/z 181)
frequently visible in perfluorinated compounds:
M-F peak
in monofluorides cleavage of α,β C-C bond is
less important (compairing to the other
monohalides)
more important: cleavage of α C-H bond


high electronegativity of F → the charge is left on
the α C-atom
more stable carbocation is formed
Aliphatic fluorides (cont.)
Benzyl halides


The molecular ion peak is
usually detectable
The benzyl (or tropylium)
ion forming by loss of
halide is favored

even more than β-bond
cleavage of an alkyl
substituent
Aromatic halides
The molecular ion peak is readily
apparent
 The M – X peak is large for all
compounds in which X is attached
directly to the ring

Heteroaromatic compounds




The molecular ion peak is intense
Cleavage of the bond β to the ring as in
alkylbenzenes, is the general rule
In pyridine the position of substitution
determines the ease of cleavage of the β-bond
cleavage occurs in such a way that charge is
usually localized on the heteroatom rather
than in the ring π-structure
Heteroaromatic compounds. Five
membered rings

Furan, thiophene, pyrrole have very similar ring
cleavage patterns:



furan - two principal peaks:


C3H3+ (m/z 39) and HC≡O+ (m/z 29)
thiophene – three peaks:


cleavage of the carbon-heteroatom bond
loss of either a neutral acetylene molecule, or radical
fragments
C3H3+ (m/z 39), HC≡S+ (m/z 45) and C2H2S+ (m/z 58)
pyrrole – three peaks:


C3H3+ (m/z 39), HC=NH+ (m/z 28) and C2H2NH+ (m/z 41)
in addition, by elimination od HCN it gives peak at m/z 40
Heteroaromatic compounds. Six
membered rings


cleavage of the β C-C bond in alkylpyridines is
more pronounced when the alkyl group is in
the position 3.
If alkyl group has more than 3 C-atoms and is
attached in the position 2:

migration of a hydrogen atom to the ring nitrogen