Mass Spectrometry it is very useful technique for the measurement of masses of isotopes, relative abundance of isotopes, molecular masses of the compounds and molecular structure. Principle Aston's mass spectrometer Dempster's mass spectrometer WOKING Comparison with C-12

2. Prof. Sohaib Hasnain
Punjab Group of Colleges, Faisalabad
CHEMISTRY

4. MASS SPECTROMETRY IS
AN ANALYTICAL
TECHNIQUE WHICH USED
TO MEASURE:
 EXACT MASSES OF ISOTOPES
 RELATIVE ABUNDANCE OF IOSTOPES
 MOLECULAR MASSES OF COMPOUNDS
 MOLECULAR STRUCTURE

5. WHAT IS MASS
SPECTROMETER/
SPECTROGRAPH?
Mass Spectrometer Is An Instrument Which Is Used To
Measure The Exact Masses Of Different Isotopes Of An
Element.

6. Aston’s Mass Spectrograph
• First Mass spectrograph was designed in 1919
• Used to identify the isotopes of an element on the basis of their
atomic masses
• Aston was awarded with noble prize for the discovery of isotopes

7. Dempster’s Mass Spectrometer
• Develped by A.J.Dempster in1918
• Used for the identification of elements which available in solid state

8. PRINCIPLE OF MASS SPECTROMETRY
In this technique, a substance is first volatilized and then
ionized with the help of high energy beam of electrons. The
gaseous positive ions, thus formed, are separated on the
basis of their mass to charge ratio (m/e) and then recorded
in the form of peaks.

9. Construction And Working Of
Dempster’s Mass
Spectrometer
Determination Of Exact Masses And Relative Abundance Of Isotopes

10. MASS SPECTROMETER

11. STEPS INVOLVED DURING THE WORKING:
Vaporization
Ionization
Acceleration
Deflection
Collection

12. Vaporization/ Volatilization
• The substance whose analysis for the separation of isotopes is
required, is converted into vapor phase
• Vaporization is done in vaporizing chamber
• The pressure kept for this step is very low 10-6 to 10-7 torr
• Solid and liquid are changed into vapor phase
• Vaporization is not required for the gases because these are already
present in vapor phase

13. Ionization
• Vapor are allowed to enter in ionization chamber
• Vapors are ionized with the help of high energy electrons
• These positively charged ions of isotopes of an element have different
masses depending upon the nature of isotope

14. Ionization
• We have sample of hydrogen for ionization
• When high energy electrons are thrown on hydrogen atoms, its
electron are knocked out
• Uni positive cation are formed because only one electron is removed
from its each atom
• Masses of ions are different because isotopes of hydrogen have
different mass
• That’s why these have differet m/e Value

16. Acceleration
• This step is required to speed up the movement of cations
• Cations are speed up by providing electric field
E= 500-2000 volts
• Greater the strength of electric field, greater will be the speed of
cations toward the negative Grid
• Now, Beam of cation is formed which is travelled in straight line

17. Deflection
• The beam of accelerated positive ions are allowed to pass through
the magnetic field
• Magnetic field is perpendicular to the direction of positive ions
• Positive ion are separated due to deflection of magnetic field
• This separation is based on m/e ratio
• Greater the value of m/e, lower will be the deflection
• lower the value of m/e, greater will be the deflection
• Heavy ions are slightly deflected and Vice versa

18. MASS SPECTROMETER

19. Deflection
Mathematical Expression
m/e = H2r/E
H = strength of magnetic field
E = strength of electric field
R = radius of circular path
Larger the m/e value, larger will be the radius
Greater the value of H, Smaller will the radius
Greater the value of E, larger will be the radius

20. Collection
• This process is done by Detector, Electrometer and ion collector
• Electrometer develops the electric current
• The strength of current measured gives the relative abundance

21. Comparison with C-12

22. Mass Spectrum
A Plot Of Data In Such A Way That M/E Of Isotopes Of An Element
Are Taken On Abscissa (X-axis) And Relative Abundance Of Ions On
Ordinate (Y-axis)