Types of peaks in mass spectroscopy. Molecular ion or parent peak. base peak. fragment ions. rearrangement ion. multiple charged ion. negative ion. metastable ion. isotopes ion.

1. SUBMITTED BY : ANUPAMA.C.P.
1 SEM M-PHARM
TYPES OF PEAKS IN MASS
SPECTROSCOPY
1

2. MOLECULAR ION OR PARENT PEAK
 When a sample substances is bombarded with
electrons of energies of 9 to15eV, the molecular ion is
produced by loss of a single electron, this will give rise
to a very simple mass spectrum with essentially all of
the ions appearing in one peak called Parent peak.
M + ê M+ + 2ê
M+ Molecular ion

3.  This M+ ion is not fragmented before collection at the
detector and is recorded at M/e value corresponding to
the molecular weight of material.
 Molecular ion peak is usually the peak of highest mass
number.
 The relative height of the peak decreases in the
following order
aromatic > conjugated olefins > allicyclic > sulphide
• If a molecule yields the parent peak due to molecular
ion ,exact molecular weight can be calculated.

4. BASE PEAK
 If an electron beam of energy of 70eV is used in a
mass spectrometer, the molecular ion is produced by
the loss of single electron which undergoes splitting to
form many fragments, the parent peak in mass
spectrum is called base spectrum.
 The largest peak in MS or most intense peak in the
spectrum whose intensity taken as 100%
 Sometimes M+ ion peak may be base peak .
e.g. Toluene

5. FRAGMENT IONS
 When potential energy is given to the M+ ion during
electron impact, further cleavage takes place and ions
of lower mass number ie, fragment ions are produced.
 Many peaks in mass spectrometer are due to
fragmentation .
 These peaks in spectrum give valuble information
regarding molecular structure.
 Fragmentation decreases in fall order
branched hydrocarbon > cooH > esters

6. REARRANGEMENT ION
 In some cases, fragments are observed which are not a
part of the original molecule. These are known as
rearrangement ions which are formed from the
molecular ion by redistribution of atoms or groups of
atoms at the moment of decomposition of the
molecular ion.
 Hydrogen transfer rearrangements are very common .
 In hydrocarbons , the rearrangement is non specific
and unpredictable.

7. MULTIPLE CHARGED ION
 In mass spectrometer, the ions are generally carrying a
single positive charge, however sometimes doubly or
even triply charged ion are found in the mass
spectrum.
 The formation of these multiple charged ions are
common in hetero aromatic atom.

8. NEGATIVE IONS
 In addition to positive ions negative ions may be
formed from electron bombardment of the sample.
 The formation of negative ions is very rare but these
can be produced in three ways.
i. AB + e A + B-
ii. AB + e AB-
iii. AB +e A+ + B- + ē
 These are not very useful in structural
determinations.
 eg,. O-, OH-

9. METASTABLE IONS
 The ions resulting from the decomposition between
the source region and magnetic analyser are called
metastable ion which appear in the spectrum as broad
peaks.
 The peaks are weaker in intensity but are useful in
studying the mechanism of fragmentation.
 These are not used for the study of structure .
 The relationship between the apparent m/e of the
metastable ion and its parent is given by the following
formulation.

10. m1
+ m2
+ + m0
The metastable ion is observed at a mass m* which is
related to m1 and m2 by the equation.
m* = m2
2
m1
m1= mass of parent ion
m2 = mass metastable ion
mo = mass of neutral fragment

11. ISOTOPES IONS
 In organic compounds, there are generally a small peak
appear at one mass unit higher than the parent peak
(M+1), Due to presence of C, H, N, O, S Isotopes.
 If the same sample contains two heavy isotopes like
Cl, Br then additional smaller peaks occur at M+2.

12. REFERENCES
 Instumental method of chemical analysis by
Gurudeep.R. Chatwal: 2012: 5th edition pg.no. 2.285-
2.287.
 Elementary organic spectroscopy principles and
chemical applications by Y.R. Sharma:4th edition ,2009
p.g.no. 288.