interpretation of mass spectroscopy.fragmentation process

1. BASIC PRINCIPLES IN
INTERPRETATION OF MASS
SPECTRA
KOMMINENI.VIDYACHOWDHARY
VAAGDEVI PHARMACY COLLEGE
BOLLIKUNTA,WARANGAL

2. MASS SPECTROMETRY
Mass spectrometry is the most accurate method for
determining the molecular mass of the compound and
its elemental composition.
It is also called as positive ion spectra or line spectra.

3. Introduction
• Mass spectrometry (Mass Spec or MS) uses high energy
electrons to break a molecule into fragmentation.
• A beam of high-energy electrons breaks the molecule apart.
• The masses of the fragments and their relative abundance
reveal information about the structure of the molecule.
• Separation and analysis of the fragments provides information
about:
– Molecular weight
– Structure

4. General approach for acquiring
information about a sample
• degree of unsaturation
• Structures and functional groups Connectivity of
structures
• Reference spectra and comparison to possible
structures where unique identification is not
possible

5. GENERAL RULES FOR INTERPRETATION OF
MASS SPECTRA
.In order to interpret the mass spectrum , one should attain
an understanding of the ionisation process.
.To observe fragmentation pattern.
1.The exact molecular weight: The molecular weight of a
pure compound from the identification of the parent peak.
The molecular weight one can determine molecular
formula.
2.The isotope effects : Heavy isotopes will exhibit peaks in a
mass spectrum at m/e one or more units higher than
normal.
i.e.,there will be small peaks at M+1 and M+2.

6. The nitrogen rule
• In organic compounds , is a relationship
between the valance and the mass of the
most common isotope for most elements.
• Even elements have an even valance.
• Odd elements have an odd valance.
• This leads to the ‘nitrogen rule.’ It assumes
that we are limiting our elements to C, H,
halogens, O and N.

7. The nitrogen rule
• A compound containing only C, H, O or X
will have an even molecular weight.
A compound with an odd number of
nitrogens will have an odd molecular weight.
• A compound with an even number of
nitrogens will have an even molecular
weight.

8. Logical losses

9. General steps in
mass spectral interpretation

10. What is a mass spectrum

11. m/z ratio:
Molecular weight divided by the
Charge on this protein
All proteins are sorted based on a
mass to charge ratio (m/z)

12. RELATIVE ABUNDANCE:
• It is a method of reporting the amount of each
mass to charge measurement after assigning the
most abundant ion 100%.
• Abundance:
. The amount of an isotope of an element that
exists in nature , usually expressed as a
percentage of the total amount of all isotopes of
the element.

13. A simple mass spectrum

14. A simple mass spectrum

15. A simple mass spectrum

16. A simple mass spectrum

17. A simple mass spectrum

18. Fragmentation pattern

19. What is base peak?
• 100% abundance is called as base peak.
• M- is parent ion peak . It is all so called as
molecular ion peak.
• Molecular peak of M produced when an
electron is ejected from the molecule
M + e- M++2e-

21. Background of fragmentation
• The impact of a stream of high energy
electrons causes the molecule to lose an
electron forming a radical cation.
– A species with a positive charge and one unpaired
electron.
+ e
-
C H
H
H
H H
H
H
HC + 2 e
-
Molecular ion
M/z=16

22. Background
• The impact of the stream of high energy electrons can also
break the molecule or the radical cation into fragments.
(not detected by MS)
m/z = 29
molecular ion (M
+
) m/z = 30
+ C
H
H
H
+ H
HH C
H
H
C
H
H
H C
H
H
C
H
H
H C
H
H
+ e
-
H C
H
H
C
H
H
H
m/z = 15

23. Background
• Molecular ion (parent ion):
– The radical cation corresponding to the mass of the
original molecule
• The molecular ion is usually the highest mass in
the spectrum
– Some exceptions specific isotopes
– Some molecular ion peaks are absent.
H
H
H
HC H C
H
H
C
H
H
H

24. Background
Mass spectrum of ethanol(MW = 46)(C2H5OH)
M+

25. Background
M+
base peak
The mass spectrum of ethanol

26. Fragmentation Patterns
• The impact of the stream of high energy
electrons often breaks the molecule into
fragments, commonly a cation and a radical.
– Bonds break to give the most stable cation.
– Stability of the radical is less important.

27. Fragmentation Patterns
• Alkanes
– Fragmentation often splits off simple alkyl groups:
• Loss of methyl M+ - 15
• Loss of ethyl M+ - 29
• Loss of propyl M+ - 43
• Loss of butyl M+ - 57
– Branched alkanes tend to fragment forming the
most stable carbocations.

28. Fragmentation Patterns
• Mass spectrum of 2-methylpentane(M.w-86)

29. Example:

30. ASPIRIN (or) SALICYLICACID(m.wt-180)

31. Mass spectra of toulene:(m/z=91)

32. Conclusion
• We’ve totally left out any of the mechanisms
of ion fragmentation.
• On the other hand, the assumption was that
you were working with a GC/MS system.
• The chromatographic information will also
help solve many problems.