Mass spectrometry is a technique that ionizes chemical compounds and separates and identifies ions based on their mass-to-charge ratios. It can provide both qualitative and quantitative information about molecular structures. During analysis, molecules are bombarded by electrons which produces molecular ions and fragment ions. These ions are then separated by their mass in electric and magnetic fields. By analyzing the masses of molecular and fragment ions, mass spectrometry can determine molecular formulas and elucidate molecular structures. It is a highly sensitive technique that is widely used for molecular identification.

1. Presented by :
Arvind Singh Heer
MSc-II
(Sem-IV)
Analytical Chemistry
Paper-II
MITHIBAI COLLEGE
MASS SPECTROSCOPY

2. CONTENT
 INTRODUCTION.
 PRINCIPLE.
 CORRELATION OF MASS SPECTRA WITH
MOLECULAR STRUCTURE.
 MOLECULAR IDENTIFICATION.
 METASTABLE PEAKS.

3. INTRODUCTION
 Mass spectrometry is one of the most
generally applicable tools providing both
qualitative and quantitative information about
the atomic and molecular materials.
 Here the compound under the investigation is
bombarded with a beam of electrons which
produce an ionic molecule or ionic fragments
of the original species. The resulting charging
particle is then separated according their
masses.

4. PRINCIPLE
 Organic molecules are bombarded with
electron and are converted to high energetic
positively charged ions(molecular ions or
parent ions).
 Which will break up in to smaller ions
(fragment ions or daughter ions).
 The loss of electron from a molecule leads to
a radical cation.
M+e- M++2e-
 This molecular commonly M+ decomposes to
a pair of fragments which may be either
radical +anion or a small molecule + radical

5.  These molecular ions, are isolated in the
electric field at an voltage V, these charging
particles which are isolated then made to
enter into an magnetic field H. Here the field
attracts the particles and move in a circle
around it.
 Here the radius of the ionised molecule
depends on m, its mass. This forms the basis
of separation of particles according to their
masses.
 These ions are made to impinge upon the
collector in turn thus giving rise to a spectrum.
 The ion source, ion path and collector of the

6. CORRELATION OF MASS
SPECTRA WITH MOLECULAR
STRUCTURE
 The ionization efficiency of a mass spectrometer
source must be high so that a large portion of the
neutral sample particles present are converted to
ions.
 High efficiency is particularly important for the
analysis of nanogram quantities of sample
material and trace impurities in solids.
 When bombarded by e- in the electron-impact
method of ionization, every substance ionizes
and fragments uniquely.
 A molecule may simply lose an e- of many
fragments into two smaller units, an ionized

7.  A molecular ion is generally observed in
considerable intensity when the gaseous molecules
are bombarded with e- of energy just sufficient to
cause ionization, but not bond leakage, about 8-
14eV for most organic molecules.
 As soon as the excess energy over the ground state
energy possessed by the molecular ion becomes
equal to the dissociation energy of some particular
bond, the appropriate fragment ions are formed.
Although the peak intensities are extremely
sensitive to ionizing voltage at low values of ionizing
voltage, the relative peak intensities become fairly
constant once the ionization voltage exceeds 50eV.
 At higher ionizing energies the total production of
ions is higher, but the net effect of higher overall
intensity and the resultant severe fragmentation is

8. MOLECULAR IDENTIFICATION
 In the identification of a compound, the most
important information is the molecular weight. The
mass spectrometer is unique among analytical
methods in being able to provide this information
very accurately, often to four decimal places when
a high-resolution mass spectrometer is used.
 At ionizing voltages ranging from 9-14V in the
electron-impact mode of ionization, it is assumed
that no ions, heavier than the molecular ion form.
Therefore, the mass of the heaviest ion, exclusive
of isotopic contributions, gives the nominal
molecular weight with lo-resolution mass
spectrometers and the exact molecular weight

9.  The no. of possible molecular formulas is
restricted by a study of relative abundances of
natural isotopes for different elements at masses
one or more units larger than the parent ion.
Observed values are compared with those
calculated for all possible combinations of the
naturally occurring heavy isotopes of the
elements. For a compound CwHxOzNy a simple
formula allows one to calculate the percent of the
heavy isotope contributions from a monoisotopic
peak, PM, to the PM+1 peak.
 100 PM+1/PM = 0.015x + 1.11w + 0.37y + 0.037z

11. METASTABLE PEAKS
 A one-step decomposition process may be
indicated by an appropriate metastable peaks in
the mass spectrum. Metastable peaks arise from
ions that decompose in the field-free region after
they are accelerated out of the ion source but
before they enter the analyzer. Their life time is
about 10^-6 sec. A metastable ion transition takes
the general form: original ion daughter
ion + neutral fragment
 The metastable peak m* appears as a weak,
diffuse peak, usually at a nonintegral mass, given
by,
m* = (mass of daughter ion)^2 / mass of

12.  In a spectrum that is linear with respect to mass
values, the distance of m*, below the daughter
ion is of similar magnitude to the distance of the
daughter ion below the original ion. The foregoing
relationship holds only for ions that decompose in
a small portion of the accelerating region and is
more frequently observed with 60 degree and 90
degree sector instruments where a field-free
region exists after the accelerating slits and
before the magnetic analyzer. Of course, the
absence of a metastable peak from the spectrum
does not preclude a particular decomposition.

13. REFRENCE
Spectroscopy
B.K Sharma
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Thank you