Mass

Spectroscopy

Dr. B. Madhu Harika

 Contents
• Introduction
• Basic principle
• Theory
• Brief outline of instrumentation.
• Ion formation and types
• Fragmentation processes
• Fragmentation patterns
• Fragmentation characteristics in relation toparent
structure and functional groups
 Mass spectroscopy
• Mass spectroscopy is one of the primary
spectroscopic methods for molecular analysis
available to organic chemist.
• It is a micro analytical technique requiring only a
few nanomoles of the sample to obtain
characteristic information pertaining to the
structure and molecular weight of analyte.
• It is not concerned with non-destructive
interaction between molecules and
electromagnetic radiation.
• It involves the production and separation of ionised
molecules and their ionic decompositon product and
finally the measurement of the relative abundance of
different ions produced. It is, thus a destructive technique
in that the sample is consumed during analysis.
• In most cases, the nascent molecular ion of the analyte
produced fragment ions by cleavage of the bond and the
resulting fragmentation pattern constitutes the mass
spectrum.
• Thus, the mass spectrum of each compound is unique and
can be used as a―chemical fingerprint to characterize the
sample.
 Basic principle
• Mass spectroscopy is the most accurate method for
determining the molecular mass of the compound and
its elemental composition.
• In this technique, molecules are bombarded with a beam
of energetic electrons.
• The molecules are ionised and broken up into many
fragments, some of which are positive ions.
• Each kind of ion has a particular ratio of mass to charge,
i.e. m/e ratio (value). For most ions, the charge is one
and thus, m/e ratio is simply the molecular mass of the
ion.
Fragments
• Mass spectra is used in two general ways:
1)To prove the identity of two compounds.
2)To establish the structure of a new compound.
• The mass spectrum of a compound helps to
establish the structure of a new compound in
several different ways:
1)It can give the exact molecular mass.
2)It can give a molecular formula or it can reveal the
presence of certain structural units in a molecule.
 Basic theory
• Mass spectroscopy deals with the examination of
the characteristics fragments (ions) arising from
the breakdown of organic molecules.
• A mass spectrum is the plot of relative
abundance of ions against their mass/charge
ratio.
• The basic aspect of organic mass spectrometry
consist of bombarding the vapour of an organic
compound with a beam of energetic electron
accelarated from a filament to an energy of 70eV
to form positively charged ions(molecular ions).
• The additional energy of the elecrons is dissipated
in breaking the bonds in the molecular ion, which
undergoes fragmentation to yield several neutral
or positively charged species.
• This fragmentation may result in the formation of
an even-electron ion and radical.
• The various positive ions, thus formed, can be
accelerated and deflected by magnetic or electric
fields.
• The deflection of ions, however, depends on its
mass, charge and velocity.
• For a given charge, velocity and deflecting force, the
deflection is less for a heavy particle as compared to
that of a lightone.
• Thus, a number of beams each containing ions with the
same m/z values are obtained.
• These beams are then made to strike against a
photographic plate where not only they appear as
separate lines but the intensity of each peak is also
recorded.
• The clear visual presentation of a mass spectum is
usually obtain by plotting m/z value against relative
abundance, assigning the most abundantion (base
peak) in the spectrum as 100 percent.
• Though organic mass spectrometry is routinely
used along with IR, NMR and UV for structure
detemination, its basic theory is different from the
others.
• In mass spectrometry no characteristic selective
absorption of radiation is involved as in the case of
the other three methods, secondly, in the mass
spectrometry, the compound undergoes
irriversible chemical changes unlike in the others,
where the changes are reversible physical changes.
• The mass spectral reactions are much more drastic
than usual chemical reactions.
Principle and Instrumentation
Ionisation
• The atom is ionised by knocking one or more
electrons off to give a positive ion. (Mass
spectrometers always work with positive ions).
• The particles in the sample (atoms or molecules) are
bombarded with a stream of electrons to knock one
or more electrons out of the sample particles to
make positive ions.
• Most of the positive ions formed will carry a
charge of +1.
• These positive ions are persuaded out into the
rest of the machine by the ion repeller which
is another metal plate carrying as light positive
charge.
Acceleration
• The ions are accelerated so that they all have the
same kinetic energy.
• The positive ions are repelled away from the positive
ionisation chamber and pass through three slits with
voltage in the decreasing order.
• The middle slit carries some intermediate voltage
and the final at‗0‘volts.
• All the ions are accelerated into a finely focused
beam.
Deflection
• The ions are then deflected by a magnetic
field according to their masses. The lighter
they are, the more they are deflected.
• The amount of deflection also depends on the
number of positive charges on the ion-The
more the ion is charged, the more it gets
deflected.
• Different ions are deflected by the magnetic
field by different amounts. The amount of
deflection depends on:
– The mass of the ion: Lighter ions are deflected
more than heavier ones.
– The charge on the ion: Ions with 2 (or more)
positive charges are deflected more than ones
with only 1 positive charge.
Detection
• The beam of ions passing through the
machine is detected electrically.
• When an ion hits the metal box, its charge is
neutralised by an electron jumping from the
metal onto the ion.
• That leaves a space amongst the electrons in
the metal, and the electrons in the wire
shuffle along to fill it.
• A flow of electrons in the wire is detected as
an electric current which can be amplified and
recorded. The more ions arriving, the greater
the current.
 Ionozation techniques

1.Gas
•gases and vapour
Phase
Ionization

2. Desorption • liquid and solid

Technique
 1.Gas Phase Ionization (gases and vapour)
• Samples are ionized outside the ion source.
• This technique include:

1.Electron impact
ionization (EIS)

2.Chemical
ionization.(Cl)

3.Field
ionization.(FI)
 2.Desorption Technique(liquid and Solid)

• Samples are ionised inside the ion source.

• This technique include:

3.Laser
ÖRSOrptîon.(LO)
 Chemical Impact Ionization
• Chemical Impact Ionization between
interactions of sample with large amount of
reagent gas.
• Commonly used reagent gases include
methane, ammonia, isobutane.
• Oxygen and hydrogen are used in Negative ion
chemical ionization in MS.
• The vaporised sample is introduced into the
mass spectrometer with an excess of a
reagent gas (methane) at pressure of about 1
torr.
• The excess carrier gas is ionized by
electron impact to the primary ions CH4 +
and CH3 .
• These may react with the excess methane to
give secondary ions
•In chemical ionization, the ionization of the analyte is
achieved by interaction of it's mokcules with ions of a
reagent gas in ie chamber or source.
•Chemical ionization is carried out in an instrument
similar to electron impact ion source with some
modifications such as:-
•Addition of a vacuum pump.
•Naroring of exit slit to mass analyzer to maintain reagent
gas pressure of about 1 torr in the ionization chamber.
•Providing a gas inlet.
•ADVANTAGES
•Used for high molecule weight compounds.
•Used for samples which undergo rapid
fragmentation in El.

•LIMITATIONS
•Not suitable for thermally unstable and non-
volatile samples.
•Relative less sensitive then El ionization.
•Samples must be diluted with large excess of reagent
gas to prevent primary interaction between the
electrons and sample molecules.
 Field Ionization
•Fl is used to produce ions from volatile compounds
•that do not give molecular ions by El.
•It produces molecular ions with little or no
fragmentation.
•Application of very strong electric field induces
emission of electrons.
•FI utilizes 10-micron diameter tungsten emitter
wires on which carbon whiskers, or dendrites, have
been grown.
•A high electric field gradient (l 0" V/cm) at the tips
of the whiskers produces ionization
•In this method the molecule pass through sharp
metal anode
• carrying an electric field of 1010v/ m
•Electrons are analysed in primary focusing Cathode
slit.
•Advantage: high abundance of molecular ions.
•Disadvantage: lower resolution.
 Field desorption
• In field desorption method, a multitipped emitter (made up of
tungsten wire with carbon or silicon whiskers grown on its surface)
similar to that used in FI is used.
• • The electrode is mounted on a probe that can be removed from the
sample compartment and coated with the solution of the sample.
• • The sample solution is deposited on the tip of the emitter whiskers
either by
– dipping the emitter into analyte solution or
– using a microsyringe.
• • The probe is then reinserted into the sample compartment which is
similar to CI or EI unit.
• • Then the sample is ionized by applying a high voltage to the emitter.
• Ionization takes place by quantum mechanical tunneling
mechanism, which involves transfer of ions from the sample
molecule to the anode (emitter).
• • This results in formation of positive ions which are radical ions
(M+) and cations attached species such as (M+Na)+.
• • (M+Na)+ are produced during desorption by attachment of trace
alkali metal ions present in analyte.
• ADVANTAGES
– Works well for small organic molecules, low molecular weight polymers
and petrochemical fractions.
• DISADVANTAGES
– Sensitive to alkali metal contamination.
– Sample must be soluble in a solvent.
– Not suitable for thermally unstable and non volatile samples.
– Structural information is not obtained as very little fragmentation occurs.
 Electron spray ionization
• Electrospray ionization is a technique used in mass
spectrometry to produce ions from macromolecules such as
proteins, polypeptides and oligonucleotides having molecular
weights of 10,000 Da or more.
• The method generates ions from solution of a sample by
creating fine spray of charged droplets.
• A solution of sample is pumped through a fine, charged stainless
steel capillary needle at a rate of few microlitres/minute.
• The needle is maintained at a high electric field (several
kilovolts) with respect to cylindrical electrode.
• The liquid pushes itself out of the capillary as a mist or aerosol
of fine charged droplets.
• In the set of aerosol droplets is produced by a process involving
formation of Taylor cone and a jet from the tip of this cone.
• These charged droplets are then passed through
desolvating capillary where the solvent is evaporated in the
vacuum and attachment of charge to the analyte molecules
takes place.
• Desolvating capillary uses warm nitrogen as nebulising gas.
• The desolvating capillary is maintained under high pressure.
• As the droplets evaporate the analyte molecules comes closer
together.
• These molecules become unstable as the similarly charged molecules
comes closer together and the droplets explode once again. This is
referred as Coulombic fission.
• The process repeats itself until the analyte is free from solvent and is
lone ion.
• The ion then moves to the mass analyzer.

• ADVANTAGES
• Most important techniques for analysis of high molecular weight
biomolecules such as polypeptides, proteins, oligonucleotides and
synthetic polymers.
• Can be used along with LC and capillary electrophoresis.
Matrix assisted laser desorption (MALDI)
Matrix assisted laser desorption is a technique in mass spectrometry for ionization of
biomolecules (polymers such as proteins, polypeptides and sugars) and synthetic
polymers that are more fragile and form fragments when ionized by conventional
methods.
• It is most similar to ESI in both softness and ions produced.
• A) Matrix
Matrix is used in MALDI to
 Absorb the laser energy.
 Prevent analyte agglomeration.
 Protect analyte from being destroyed by direct laser beam.
 Matrix consists of a crystallized molecules of which the most commonly used are :-
3,5 – dimethoxy – 4 – hydroxy cinnamic acid (sinapinic acid)
α – cyano – 4 – cinnamic acid (α – cyano or α – matrix)
2,5 – dihydroxy benzoic acid (DHB)
• Preparation of matrix:
– Solution of the matrix is made in a mixture of highly purified water and another organic
compound (acetonitrile or ethanol).
– b) Triofluoro acetic acid (TFA) is also added.
– If sinapinic acid is used as a matrix the solution is prepared by adding 20 mg/ml of sinapinic
acid, Water: acetonitrile: TFA (50:50:0.1)
– Matrix solution is then mixed with the analyte to be investigated.
– The solution is then spotted in a air tight chamber on the tip of the sample probe.

• With a vacuum pump the air is removed and vacuum is created which leads to
evaporation of the solvent leaving behind a layer of recrystalized matrix containing
analyte molecules.
Some of the more commonly used matrices are:
• B) Laser
– The solid mixture is then exposed to pulsed laser beam.
– The matrix absorbs the laser energy and transfers some of this energy to the
analyte molecules which results in the sublimation of sample molecules as
ions or the matrix after absorbing the laser energy gets ionized and transfer
part of this charge to the sample molecules and ionize it.
– Nitrogen or carbon lasers are most commonly used.

• The ions produced in this process are quassimolecular ions that are
ionized by addition of proton (M+H) or a cation such as sodium
+

(M+Na) or removal of a proton (M-H) .

+ -

• It generally produces singly charged ions in some cases doubly charged

ions such as (M+2H) are also observed.
2+

• The chamber consists of two electrodes and the ions are produced
between the electrodes.
• When the polymers form cations the cathode is placed right behind
the sample and anode in front of the sample.
• The cations get attracted towards the negatively charged anode.
This acceleration is used to move the ion to the detector.
• When the polymer forms anions the electrodes are interchanged.

• APPLICATIONS
• Used in proteomics
• Estimation of DNA, RNA and oligosaccharides.
• Used in analysis of lipids, phosphopeptides and synthetic polymers.
 Plasma desorption

• Plasma desorption produces molecular ions from the samples

coated on a thin foil when a highly energetic fission fragments from
the Californium-252 “blast through” from the opposite side of the
foil.
• The fission of Californium-252 nucleus is highly exothermic and the
energy released is carried away by a wide range of fission
fragments which are heavy atomic ion pairs.
• Ion pair fission fragments depart in opposite directions.
• Each fission of this radio active nucleus gives rise to two fragments
traveling in opposite directions (because necessity of momentum
conversation).
• A typical pair of fission fragments is 142Ba18+ and 106TC22+, with kinetic
energies roughly 79 and 104 MeV respectively.
• When such a high energy fission fragments passes
through the sample foil, extremely rapid localized
heating occurs, producing a temperature in the range of
10000K.
• Consequently, the molecules in this plasma zone are
desorbed, with the production of both positive and
negative ions.
• These ions are then accelerated out of the source in to
the analyzer system.
 Fast Atom Bombardment (FAB)

• It is an ionization technique in which the analyte and non-volatile liquid

matrix mixture is bombarded by a high energy beam of inert gas such as
Argon or Xenon.
• This technique is used for ionization of polar high molecular weight
compounds such as polypeptides.
• Commonly used matrices include :-
– Glycerol
– Monothioglycerol
– Carbowax
– 2,4 – dipentyl phenol
– 3 – nitrobenzyl alcohol (3 – NBA)
• These solvents easily dissolve organic compounds and do not evaporate in
vacuum.
CONSTRUCTION & WORKING:

• The bombarding beam consists of Xenon or Argon atoms of high

translational energy.
• This beam is produced by first ionizing the Xenon (or Argon atoms with
electrons to give Xenon radical cations.
Xe + e - = Xe.+ +2e-
• The radical cations are then accelerated to 6 – 10 KeV to give radical cations
of high translational energy (Xe)++, which are then passed through a
chamber containing Xenon atoms at a pressure of 10 -5 torr.
• During this passage high energy cation obtain electrons from Xenon atoms
to become high energy atoms (Xe).
• The lower energy ions are removed by electrostatic deflector.
(Xe)++ Xe.+ + Xe
(Xe).+ + Xe (Xe) + Xe.+
MATRIX PREPARATION:

• The analyte is dissolved in the liquid matrix such as glycerol and applied
as a thin layer on the sample probe shaft.
• The mixture is bombarded with the high energy beam of Xenon atoms.
• Xenon ionizes the glycerol molecules to give glycerol ions.
• These ions react with the surrounding glycerol molecules to produce
(G+H)+ as reactant ions.
• The sample molecules then undergo proton transfer or hydride transfer
or ion-pair interaction with reactant ions to give quassimolecular or
psuedomolecular ions such as (M+H) +, (M-H)- or (M+G+H)+.
• These ions are then extracted from slit lens system designed to collect
ions and directed to mass analyzer.
• ADVANTAGES
• Used for ionization of polar high molecular weight samples.
• Provides rapid heating of samples and reduces sample
fragmentation.
• Rapid ionization.
• DISADVANTAGES
• Difficult to distinguish between low molecular weight
compounds.
• Compounds must be soluble in liquid matrix.
• Not good for multiply charged compounds.
 Mass Analyzers
• Quadrapole Mass Analyzer:
• The quadrupole mass spectrometer is a scanning mass
analyzer that uses the stability of ion trajectories in
oscillating electric fields to separate them according to their
mass-to-charge ratios (m/z).
• It comprises four rods arranged parallel to each other and a
constant DC potential is applied to each rod pair, one pair
positive and the other negative, on opposite planes.
• An alternating potential with frequency in the region of
radio waves is superimposed over the DC potential on each
rod pair. This radio frequency (RF) voltage causes the ions to
spiral as they traverse the quadrupole toward the detector.
• This arrangement acts as a high pass m/z filter
in one plane (+), and a low pass m/z filter in the
other plane (–).
• Typically, the DC and RF potentials are adjusted
so that only one m/z at a time can traverse the
quadrupole.
• All others collide with the rods and are
neutralized.
• To acquire a mass spectrum, the DC and RF
potentials must be scanned in constant ratio
across a range. Therefore, acquiring a broader
mass range requires a longer scan time.
• The advantages of a quadrupole system
include its low cost, ability to perform both
qualitative and quantitative analyses, and
increased sensitivity capability through
selected ion monitoring (SIM) mode.
• The major cons of quadrupole analyzers are
that their scanning nature limits the
acquisition rate and leads to spectral bias.
 TOF-MS
• A time-of-flight (TOF) mass spectrometer is a nonscanning
mass analyzer that emits pulses of ions (or transients) from
the source.
• The ions are accelerated so that they have equal kinetic
energy before entering a field free drift region, also known
as the flight tube.
• Because kinetic energy is equal to ½ mv2, where m is the
mass of the ion and v is the ion velocity, the lower the ion's
mass, the greater the velocity and shorter its flight time.
• The travel time from the ion source through the flight tube
to the detector can be transformed to the m/z value
through the relationships described previously
• Because all ion masses are measured for each transient,
TOF mass spectrometers are well-suited for the analysis
of both targeted and nontargeted analytes.
• A nonscanning instrument such as a TOF-MS system
offers many advantages including fast acquisition rates,
spectral continuity, and exceptional dynamic range.
• The ability to acquire full mass range spectra without
sacrificing speed or sensitivity makes TOF-MS an
excellent choice for qualitative and quantitative
analyses across a wide dynamic range in the presence
of complex matrices.
 Fragmentation process:
• Bombardment of molecules by an electron
beam with energy between 10-15 ev usually
results in the ionization of molecules by removal
of one electron (Molecular ion formation).
• When the energy of electron beam is increased
between 50-70 ev, these molecular ions acquire
a high excitation resulting in their breakdown
into various fragments. This process is called ―
Fragmentation process.
Types of Ions:
1. Molecular ion or Parent ion.
2. Fragment ions.
3. Rearrangement ions.
4. Multicharged ions.
5. Negative ions.
6. Metastable ions.
 Molecular ion or Parent ion:
• When a molecule is bombarded with electrons in
high vacuum in Mass spectrometer, it is converted
into positive ions by loss of an electron.
• These ions are called as Molecular or Parent ions.
M + e- M+ + 2e—
Where, M represents the Molecule;
M+ represents the Molecular or Parent ion.
The order of energy required to remove electron is as
follows—
σ electrons > non-conjugated π > conjugated π > non
bonding or lone pair of electrons.
• Molecular peak is observed if molecular ion
remains intact long enough (10-6seconds) to
reach the detector. This peak gives the molecular
weight of the compound. The molecular ion peak
is usually the peak of the ―highest mass number.
• Significance of Molecular ion:
– Molecular ion peak gives the molecular weight of the
compound.i.e. m/z of molecular ion = molecular
weight of the compound.Ex: C2H5+(m/e=29) gives the
molecular weight of Ethane.
Fragment ions:
• When the energy is given to Molecular ion
during electron impact, further cleavage takes
place and ions of lower mass number known
as Fragment ions are produced.
M+ M+1 + M2
• Ex: CH3OH+ CH2OH+ + H
m/z32 m/z31
Rearrangement ions:
• Rearrangement ions are the fragments whose
origin cannot be described by simple cleavage of
bonds in the parent ion, but are result of
intramolecular atomic rearrangement during
fragmentation.
• These are probably due to recombination of
fragment ions and known as rearrangement peaks.
Ex:Prominent peak in spectrum of diethyl ether
occurs at m/e31. This is due to the ions CH3O+,
which is formed by rearrangement of C2H5O+ ions.
 Multicharged ions:
• Sometimes ions may also exist with two or three
charges instead of usual single charge in the mass
spectrum. These are known as doubly or triply
charged ions. They are created as follows:
M+ + e- M ++ + 3e-
• But under normal operating conditions, most of the
ions produced are single charged. The doubly or triply
charged ions are recorded at a half or one third of the
m/e value of the single charged ions.
• Formation of these multiple charged ions is more
common in hetero Aromatic compounds. They are
also common in inorganic mass spectrum..
 Negative ions:
• The positive ions predominate in electronic
impact ionization because of greater stability.
The Negative ions are not very useful in
structural determinations. The formation of
Negative ions is very rare but these can be
produced in three ways:
1. AB + e- A + + B—
2. AB + e- AB—
3. AB + e- A + + B — + e-
 Metastable Ions:
• Fragment of a parent ion will give rise to a new ion
(daughter) plus either a neutral molecule or a
radical.
M1+ M2+ + non charged particle
An intermediate situation is possible; M1+ may
decompose to M2+ while being accelerated. The
resultant daughter ion M2+ will not be recorded at
either M1 or M2, but at a position M* as a rather
broad, poorly focused peak. Such anion is called a
metastable ion.
 General rules for fragmentation:
1. 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.
3. Cleavage is favoured at alkyl substituted carbon atoms; the
more substituted, the more likely is cleavage. This is a
consequence of the increased stability of a tertiary carbon
atom over a secondary, which in turn is more stable than a
primary.
CH3+< RCH2+< R2CH+< R3C+
 STEVENSONS RULE:
• When an ion fragments, the positive charge will remain on the
fragment of lowest ionization potential.
• Generally the largest substituent at a branch is eliminated most
readily as a radical, presumably because a long chain radical can
achieve some stability by delocalization of the lone electron.
• Ex- cleavage of 1-methyl pentane

• In this fragmentation, positive charge remains on the more high

substituted fragments, i.e. the one with lower ionization
potential.
4. Double bonds, cyclic structures and
especially aromatic or hetero aromatic rings
stabilize the Molecular ion and thus increase
the probability of its appearance.
5. Double bonds favour allylic cleavage and give
the resonance stabilized allylic carbonium.
Ex: Mass spectrum of 1-butene
6. Saturated rings tend to lose alkyl side chains
at the α carbon atom. This positive charge
tends to stay with the ring fragment.
Ex: Mass spectrum of n-propyl cyclo hexene
7. In alkyl substituted aromatic compounds,
cleavage is very probable at the bond to the
ring, giving the resonance stabilized benzyl
ion or more likely, the tropylium ion:
Ex: mass spectra of n-butyl benzene.
8. Cleavage is often associated with elimination
of small, stable, neutral molecules such as
carbonmonoxide, olefins, water, ammonia,
hydrogensulphide, hydrogencyanide,
mercaptans, ketone, or alcohols, often with
rearrangement.
Factors influencing Fragmentation process:

1. Bombardment energies
2. Functional groups
3. Thermal decomposition
 General modes of fragmentation:
• Fragmentation of the molecular ion takes place in
following modes:
1. Simple cleavage
– 1.Homolytic cleavage
– 2.Heterolytic cleavage
– 3.Retro Diels- Alder reaction
2. *Rearrangement reactions accompanied by transfer of
atoms.
– 1.Scrambling
– 2.McLaffertyrearrangement
– 3.Elimination
Homolytic cleavage :
• Here fragmentation is due to electron
redistribution between bonds.
Heterolytic cleavage:
• Fragmentation by movement of two electrons:
In this type of cleavage both the electrons of
the bond are taken over by one of the atoms;
the fragments are an even electron cation and
a radical with the positive charge residing on
the alkyl group
 Retro Diels-Alder reaction:
• Elimination by multiple σbond rupture:
• Cyclohexene is broken down to Diene and
Dienophile. It can be explained by electron
mechanism.
Rearrangement reactions accompanied by transfer of atoms:

• 1.Scrambling: Fragmentation giving rise to stable

carbocation:
• In certain cases fragmentation takes place at bond,
which gives stable carbocation.
• Ex-Molecular ion from the alkyl benzene undergoes
fragmentation at the benzylic bond and final product
is seven membered cyclic ion known as Tropylium ion
2. Mc Lafferty rearrangement:Fragmentation due to
rearrangement of Molecular or Parent ion:
• Here cleavage of bonds in Molecular ion is due to the
intramolecular atomic rearrangement.
• This leads to fragmentation whose origin cannot be
described by simple cleavage of bonds.
• When fragments are accompanied by bond formation as
well as bond for breaking, a rearrangement process is said
to have occurred.
• Such rearrangement involves the transfer of hydrogen from
one part of the molecular ion to another via, preferably, a
six-membered cyclic transition state.
• This process is favoured energetically because as many
bonds are formed as are broken.
• Compounds containing hydrogen atom at position
gamma to carbonyl group have been found to a
relative intense peak.
• This is probably due to rearrangement and
fragmentation is accompanied by the loss of neutral
molecule.
• This rearrangement is known as McLafferty
rearrangement.
• The rearrangement results in the formation of
charged enols and a neutral olefins.
• To undergo McLafferty rearrangement, a
molecule must posses
– a.An appropriately located hetero
atom(ex.oxygen)
– b.A double bond
– c.An abstractable Hydrogen atom which is
γ(gamma) to C=Osystem.
• Elimination:Fragmentation due to loss of
small molecule:
• Loss of small stable molecules such as H2O,
CO2, CO,C2H4 from molecular ion during
fragmentation.
• Ex-An alcohol readily looses H2O molecule
and shows a peak 18 mass units less than the
peak of molecular ion.