Fragmentation rules mass spectroscopy

PRESENTED BY-
SANTHOSH KUMAR T S
M.PHARM (PH ANALYSIS)
1ST YEAR
A PRESENTATION ON
MASS FRAGMENTATION AND ITS RULES
KARNATAKA COLLEGE OF PHARMACY
BANGALORE
FACILITATED TO-
Dr. T SREENIVAS RAO
ASSISTANT PROFESSOR
DEPT. OF ANALYSIS

• Mass fragmentation and its rules
HIGHLIGHTS

Mass fragmentation and
its rules

Ionizer
Sample
+
_
Mass Analyzer Detector
BLOCK DIAGRAM OF MASS SPECTROMETRY

General mode of fragmentations:
1. Simple cleavage
• Homolytic cleavage ( alpha cleavage )
• Heterolytic cleavage ( sigma cleavage )
• Retro diels alders reaction
2. Rearrangements reactions
• Mc lafferty rearrangement
• Elimination reactions

---- HC – C – Z ---- ---- C=C
+ HZ
+
Retro Diels-alder
+ .
+
.
CH2
CH2
CH2
CH2
+
+ . +
McLafferty
Z
H
Z R
CH2
CH2
Z
H
Z R
+
.
.

RULES OF FRAGMENTATION
1. Stevensons rule
2. Nitrogen rule
4. Mclafferty rearrangement
5. Retro diels alder reaction

Fragmentation rules in MS
1. Intensity of M.+ is Larger for linear chain than for
branched compound
2. Intensity of M.+ decrease with Increasing M.W.
(fatty acid is an exception)
3. Cleavage is favored at branching
 reflecting the Increased stability of the ion
Stability order: CH3
+ < R-CH2
+ < R
R
CH+ < RC+
R
R
R
R”
CH
R’
Loss of Largest Subst. Is most favored

Rule 3:
Alkanes
Cleavage Favored at branching
Loss of Largest substituent
Favored
Rule1: intensity of M
.+ is smaller
with branching
Illustration of first 3 rules (largeMW)

Illustrationof first3 rules(largeMW)

CH3
CH3
CH3
CH3
CH3
CH3
CH3
MW=170
Fragmentation occur at branching: largest fragment loss
Branched alkanes

4. Aromatic Rings, Double bond, Cyclic structures stabilize M
.+
and increases the probability of its appearance
5. Double bond favor Allylic Cleavage
 Resonance – Stabilized Cation
CH2
+
CH CH2 R
- R
.
CH2
+
CH CH2
CH2 CH CH2
+

Cycloalkane
ring has stable
M.+

6. a) Saturated Rings lose a Alkyl Chain (case of
branching)
b) Unsaturated Rings  Retro-Diels-Alder
CH2
CH2
CH2
CH2
+
+ . + .
R
+ .
+
-R
.

7. Aromatic Compounds Cleave in b  Resonance Stabilized Tropylium
C
CH
+
R
-R
.
CH
+
CH2 CH2
+
+Tropylium ion
m/z 91

8. C-C Next to Heteroatom cleave leaving the charge on the Heteroatom
R CH2 CH2 Y R
x
CH2 Y R
+
CH2
+
Y R
x
R2
C
R1
O
C
R1
O
+
C
+
R1
O
- [RCH2]
- [R2]
larger

9. Cleavage of small neutral molecules
(CO2, CO, olefins, CN, NH2 ,SH , H2O ….)
Result often from rearrangement
McLafferty
x
CH2
CH2
H
CH2
O
C
Y
Y  H, R, OH, NR2
Ion Stabilized
by resonance
x
CH2
CH2
H
CH2
O
C
Y
- CH2=CH2
x
CH2
O
C
Y
H

APPLICATIONS OF MASS SPECTROSCOPY
QUALITATIVE ANALYSIS
Determination of Molecular weight.
Determination of Molecular formula.
Determination of Partial molecular formula.
Identification of compounds from fragmentation patterns.

QUANTITATIVE APPLICATIONS
Quantitative analysis of mixtures.
Component analysis.
Gas analysis.
Isotope abundance measurement.
Isotope measurement.
Component type determination.
Thermodynamic studies.
Ion molecule reaction.
Measurement of ionisation potential.
Molecular Structure.
Gas chromatograph eluent identification.
Impurity detection.

Qualitative Applications
 The mass spectrum of a pure compound provides valuable
information of qualitative identification purposes.
 Components of mixture can also be identified by making use of
mass spectroscopy.
1. Molecular weight determination
 The molecular weight of compound that can be easily
volatilized can be calculated easily and the method requires
the identification of the molecular ion peak, the mass of which
gives the molecular weight to atleast the nearest whole
number.

 The determination of molecular weight, however, suffers from
the following difficulties:
 The molecular ion peak may either be absent or so small
that it is confused with a peak caused by an impurity
 Collision processes may produce M+1 peak that is more
intense than the parent ion peak.
2. Determination of Molecular Formula
 If it is possible to identify the molecular ion peak, the
molecular formulae can be determined from the mass
spectrum either partially or exactly.
 By making use of high resolution instrument capable of
detecting mass differences of a few thousand of unit, it is
possible to derive a unique formula for compound from the
exact mass of the ion peak.

3. Determination of Partial Molecular formula
 If a molecule contains any number of chlorine, bromine,
silicon or sulphur atoms, this fact is immediately apparent
from the mass spectrum and number of such atoms can be
rapidly and accurately deduced.
 The number of carbon atoms present can also be rapidly
estimated to ±1 atom.
 It is also possible to determine the number of oxygen and
nitrogen atoms theoretically but the values obtained are not
reliable.

4. Identification of compounds from
Fragmentation Patterns
 Fragmentation of even simple molecules produces a large
number of ions with different masses.
 A complex spectrum is obtained which is very useful for the
identification of compound or for the recognition of the
presence of functional groups in compounds.
 A careful study of the fragmentation patterns for pure
substances provides useful information regarding rational
fragmentation mechanisms and also provides a series of
general rules that are helpful in interpretation of the spectra.

Quantitative Applications
The basic requirements for a successful mass spectrometric
analysis are
 Each component must exhibit atleast one peak that
differs markedly from the others.
 The contribution of each component to a peak, must be
linearly additive.
 The sensitivity must be reproducible to perhaps one
percent relative.
 For calibration, suitable standard must be available.

1. Quantitative analysis of Mixtures
 In the quantitative analysis of mixture, the spectra are
recorded for each component.
 It should be noted that samples of each compound must be
available in a fairly pure state.
 From a careful inspection of the individual mass spectra,
known or suspected to be present in the mixture, it is possible
to select analysis peaks on the basis of the intensity and
freedom from interference by the presence of both
components.

2. Component analysis
 Large number of compounds, such as natural gas, C3-C5
hydrocarbons, C6-C8 saturated hydrocarbons, C1-C5 alcohols,
aldehydes and ketones, C1-C4 chlorides and iodides,
fluorocarbons, thiophenes and many others can be analysed
without sample heating.
 Similarly, C16-C27 alcohols, aromatic acids and esters,
steroids, fluorinated polyphenyls, aliphatic amides,
halogenated aromatic derivatives etc., can be successfully
analysed by using higher temperatures.
 High molecular weight polymeric materials have also been
analysed and characterised by Mass Spectroscopy.

3. Gas Analysis
 If the satisfactory calibration mixtures of known composition
are available, almost any type of gas mixture can be analysed
quantitatively in the case of simple mixture.
 Mass spectroscopy has widely been used in the following
typical types of gas analysis
 Monitoring process streams.
 Synthetic gas mixtures.
 Atmospheric gases, especially the noble gases, for
which it is difficult to find other methods.
 Monitoring rapid changes in gas composition

4. Isotope abundance measurement
 Mass spectrometry was originally developed for the study of
isotope abundance, and it is also used for the same purpose
in recent years.
 Information regarding the abundance of various isotopes is
now employed for a variety of purposes. Some important
applications include
 Determination of formula of organic compounds
 Analysis by isotopic dilution
 Tracer studies with isotopes
 Dating of rocks and minerals by isotopic ratio
measurements

5. Isotope measurement
 Isotopic ratios can be successfully determined accurately by
making use of a system in which two collector electrodes are
employed to collect the pair of isotopes under test.
 It should be noted that peaks due to naturally occurring
isotopes are present in any mass spectrum, and the highest of
the peaks due to an isotope relative to the height of the peak
due to the naturally most abundant ion, gives a measure of
the abundance.

6. Thermodynamic Studies
 Mass spectroscopy has also been found to be very useful in
thermodynamic studies and can be used over a wide range of
temperatures.
 Heats of vapourisation of high temperature materials can be
determined.
7. Measurement of Ionisation Potential
 Ionisation potential of a molecule is defined as the amount of
energy needed to form a molecular ion (i.e. to remove an
electron)

8. Molecular Structure
 The nature of fragments produced in the ion source of a mass
spectrometer may be made use of in the study of molecular
structure
9. Gas Chromatograph Effluent Identification
 Complex mixtures can be easily separated by making use of
gas chromatograph, but it is not possible with it to identify a
component in the effluent stream.
 It is however, possible to identify each component that is
eluted by passing all or some of the effluent into a mass
spectrometer.

REFERENCES
• www.google.com
• Instrumental method of chemical analysis – B K SHARMA
• Principles of instrumental analysis by DONGLAS SKOOG
• Instrumental method of analysis by CHATWAL

Fragmentation rules mass spectroscopy

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