SPECTROMETRY

MASS SPECTROMETRY 

A technique which is used to determine the exact relative atomic masses and relative abundances of the different isotopes of an element is called mass spectrometry.

Mass spectrometer:

An instrument which is used to determine the exact relative atomic masses and relative abundances of the different isotopes of an element is called mass spectrometer.

Principle of mass spectrometry

In mass spectrometry, 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. Actually mass spectrum is the plot of data in such a way that (m/e) is plotted as abscissa (x-axis) and the relative abundance of ions as ordinate (y-axis).

Types of mass spectrometer:

It has following types.

(i) Aston’s mass spectrometer was designed to identify the isotopes of an element on the basis of their atomic masses. It helps to identify the isotopes of the elements in gaseous state only.

(ii) Dempster’s mass spectrometer was designed to identify elements which were available in solid state.

(iii) Modern mass spectrometer: It contains two additional components than given in the Dempster’s Mass Spectrometer (amplifier and recorder).

Components of mass spectrometer:

The components of Dempster’s mass spectrometer are as follows:

(i) Vaporization or volatilization chamber.

(ii) Ionization chamber.

(iii) Electric field (accelerator)

(iv) Magnetic field (deflector)

(v)  Electrometer or Ion collector (detector)

(vi)  Amplifier

(vii) Recorder

Effect of strength of electric field (E) on deflection of Cation:   

If E is increased by keeping the H constant then

• Cations will undergo lesser deflection.                                                                   
• Radius of curvature will increase.

As a result, cation of particular m/e ratio will fall at different place as compared to first place.

Effect of strength of magnetic field (H) on the deflection of cation:

If H is increased by keeping the E constant then

• Cations will undergo more deflection
• Radius of curvature will decrease.

As a result, cation will fall at different place as compared to first place.

Mass spectrograph

Other methods of separation of isotopes:

The separation of isotopes can be done by the methods based on their properties. Some important methods are

(i) Gaseous diffusion

(ii) Thermal diffusion

(iii) Distillation

(iv) Ultracentrifuge

(v) Electromagnetic separation

(vi) Laser separation