1. Mass spectroscopy is used to determine molecular weights and formulas, identify functional groups within molecules, and identify analytes through comparison of mass spectra. 2. Samples are bombarded with electrons, which causes ionization and fragmentation into molecular and daughter ions. Ions are separated by mass and detected. 3. High resolution can be achieved through double focusing mass spectrometers, which use both electric and magnetic fields to separate ions before detection.

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PRINCIPLES OF MASS SPECTROSCOPY
Facilitated by
Dr.B.M.Gurupadayya
Head of the department
J.S.S College of Pharmacy
Mysore
Presented by
A.Shravya
Pharmaceutical Analysis
J.S.S College of Pharmacy
Mysore

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Mass spectroscopy
Organic chemists use mass spectroscopyin 3 principle ways
 To measure relativemolecularmasses (molecular weights) with high accuracy.
From this we can get exact molecular formulae.
 To detect within a molecule the places at which it prefers to fragment. We can
deduce the presence of recognizable groupings within the molecule.
 As a method for identifying analytes by comparison of their mass spectra.
Basicprinciple:
Organic molecules are bombarded with electrons and are converted to highly energetic
positively charged ions (molecularionsor parentions).
Which can break up into smaller ions (fragmentionsor daughterions ).
This loss of electron from a molecule leads to a radical cation.
M M+.
This molecular ion commonly M+. decomposesto a pair of fragments which may be either
radical + anion or a small molecule + radical cat ion.
 M+. m1
+ + m2
The molecular ions, the fragment ions and the fragment radical ions are separated by the
deflection in a variable magnetic field according to their mass and charge and generate
current (the ion current) at the collector in proportion to their relativeabundances.
o The relative mass spectrum is a plot of relative abundance against the
mass/charge (the m/z value).
o The singly charged ions, the lower the mass the more easily the ion are
deflected in the magnetic field.
o Doubly charged ions are occasionally formed these are deflected twice
as much as the singly charged ions of the same mass and they appear in

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the mass spectrum at the same value as do singly charged ions of half
the masses, since 2m/2z=m/z.
o Neutral particles produced in the fragmentation, whether uncharged
molecules (m2) or radicals (m2
.), cannot be detected directly in the mass
spectrometer.
The sample of organic compound in vapour state is bombarded with high energy
electron beam (electron impact70ev).
This ejects one of the electrons from one molecule to molecular ion (M+).
This further produces a series of fragment ions or daughter ions.
These ions are separated by deflection in a variable magnetic field according to their
relative m/z values.
The mass spectrum of a compound can be obtained on a smaller sample size (down to 10-12g)
than any of the other spectroscopic methods.
A simplified line diagram representation of the mass spectrum of 2-methylpentane (C6H14).
The most abundant ion has an m/z value of 43 (correspondingto C3H7 ), showing the most
favoured point of rupture occurs between Cx and Cy. The most abundant ion (the base peak) is
given an arbitrary abundance of 100 ,and all other intensities are e expressed as %of relative
abundances.
The small peak at m/z 86 is obviously the molecular ion.
The peaks at m/z 15, 29, 71 correspond to CH3
+, C2H5
+ and C5H11
+ respectively.
Fragment ions arise from the rapture of molecular ion either directly or indirectly and the
analysis of many thousands of organic mass spectrahas led to comprehensive semi empirical
rules about the preferred fragmentation about the preferred fragmentation modes of every kind
of organic molecules.

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Simplified mass spectrum of 2-methyl pentane

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High resolution double focusing mass spectrometer

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Instrumentation
Sample insertion
Organic compounds that have moderate vapour pressures at temperatures upto around 3000C
9including gases are placed in an ampoule connected via a reservoir to the ionization chamber.
Depending upon the volatility it is possible to coolor heat the ampoule in order to control the
rate at which it will diffuse slowly through the sinter into the ionization chamber.
Samples of lower vapour pressures are directly inserted into the ionization chamber on the
ionization chamber on the end of the probe and their volatilization is controlled by heating the
probetip.
Ion production in the ionization chamber
Electron bombardment is routinely used.
Organic molecules react on electron bombardment into 2 ways
Electron is captured by the molecule giving a radical anion.
M e M-.
Electron is removed from the molecule giving a radical cation.
M -e M+. +2e
Most organic molecules form molecular ions (M+.) when the energy of electron beam reaches
10-15ev
While this minimum ionization potential is of great theoretical importance,fragmentation ofthe
molecular ion only reaches substantial proportions at higher bombardment energies and 70ev is
used for most organic works.
When the molecular ion have been generated in the ionization chamber they are expelled
electrostatically by means of a low positive potential on a repeller plate(A) in the chamber.

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Once out the are accelerated down the ion tube by
The much higher potential between the accelerating plates B and C
Separationof ions in the analyser
In the magnetic analyzer ions are separated on the basis of m/z values
The kinetic energy E of an ion of mass m travelling with velocity v is given by
E=1/2 mv2
The potential energyof an ion of charge z is being repelled by voltage of zV
when the ions are repelled, then potential energy (zV) is converted to kinetic energy1/2 mv2..
zV=1/2 mv2 or v2=2zV/m.--------------------------1
when ions are shot into magnetic field of the analyzer they are drawn into circular motion by
the field, and at equilibrium the centrifugal force (mv2/r) is equalled by the centripetal force
exerted on it b the magnet(zBv)
mv2/r=zBV
V=zBr/m------------------------------2
Combining 1 and 2
V2=[zBr/m]2
2zV/m=z2B2r2/m2
m/z=B2r2/2V
from the equation we can see the ability of mass spectrometer to distinguish between an ion m+
ad an ion 2m+ since the ratio m/zbetween them has the same value of (B2r2/2V). And the
parameters B,r,Vdictate the path of ions.

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To change the path of the ions so that they will focus on the collector and be recorded we can
vary either
which is the accelerating voltage or B (the strength of focusing magnet)
voltage scans are more affected than the magnetic scansand are used where fast scan speed is
desirable.
Resolution
The ability of the mass spectrometer to separate two ions (the spectrometer resolution).
It is defined by measuring the depth of the valleys between the peaks produced by the ions.
A valley between them is 10% difference between them.
Simple magnetic focusing instruments have resolving powers of around 1 to 7500 .
In double focusing instruments ions repelled by the instrument donothave identical kinetic
energies and the energy spread constitutes the principal limiting factor in improving the
resolution of a magnetic analyzer.
Preliminary focusing can be carried out by passing the ion beam between the two curved
plateswhich are electrostatically charged an electrostatic analyzer.
mv2/r=zE
v2=zEr/m--------------------------------------------------3
1=3
2zV/m=Er/m
2V/E=r
Thus the radial path followed by ions of a given velocity is independent of m and z
The electrostatic analyzer focuses ions of identical kinetic energy onto the monitor slit
whatever their m/z values and coupled with a magnetic analyzer to resolve m/z vales this
double focusing spectrometer can attain the resolution of 1 to 60,000.

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The Detector–Recorder
The focused ion beam passes through the collector slit to the detector, which must convert the
impact of a stream into an electrical current.This must be amplified and recorded, either
graphically or digitally.
Disadvantages
1. This is destructive nature of process which precludes the recovery of sample.
2. It does not measure the interaction of molecules with the spectrum of energies found in
the electromagnetic spectrum, but the output from instrument has all the other
characteristics in showing an array of signals correspondingto spectrum of energies.
Resolution(R)of the instrument:
It is defined as the ability of a mass spectrometer to two ions is acceptable defined by
measuring the depth of the valleys between the peaks are produced by the ions
Consider two adjacent peaks of approximately equal intensity.
These peaks are to be chosen so that the height of the valley between the peaks is less than
10% of the intensity of the peaks.
The resolution (R) is
Where Mn = higher mass number.
Mm= lower mass number.

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The important class of mass spectrometers can measure the mass of the ion with sufficient
accuracy to determine its atomic composition.
Mass spectrometers for structure elucidation can be classified according to the method of
separating the charged partic
Magnetic Field Deflection
1. Magnetic Field only (Unit Resolution).
2. Double Focusing (High Resolution, Electric field and Magnetic field).
Quadra pole Mass Spectrometry
1. Quadrapole Mass Filter
2. Quadrapole Ion Storage (Ion Trap).
Time of Flight
Fourier Transform-Ion Cyclotron Resonance(FT-ICR)or Ion Cyclotron Resonance
Tandem Mass Spectrometry.
References
Organic spectroscopy by William Kemp
Spectrophotometric identification of organic compounds by Robert M
SILVERSTEIN and Francis X.WEBSTER.