RELATIVE ABUNDANCE OF ISOTOPES:DETERMINATION OF RELATIVE ABUNDANCE AND ATOMIC MASS OF ISOTOPES:

 RELATIVE ABUNDANCE OF ISOTOPES:

            The occurrence  of an isotopes of an element relative to other isotopes of the same element is called relative abundance  of the isotopes . Sometimes it is expressed as %age composition or sometimes it is represented in numbers. e.g;

 

Hydrogen

                        ¹H₁ = 99.985%                       ²H₁ = 0.015%

 

Chlorine        ³⁵Cl₁₇ = 75%                           ³⁷Cl₁₇ = 25%

 

DETERMINATION OF RELATIVE ABUNDANCE AND ATOMIC MASS OF ISOTOPES:

            Relative abundance and atomic mass of isotopes of an element is determined by a technique called by a technique called mass spectrometry. The instrument used in this technique is called mass spectrometer.

 

MASS SPECTROMETER:

            The instrument used to separate positively charged particles on the basis of their m/e values is called spectrometer. In order to obtain the mass and the percentage abundance for the isotopes of an element, a method required the following essential features.

 

(i)       VAPOURIZATION:

            The substance whose analysis is required for the separation of isotopes is covered into vapours. The pressure of vapours is reduced to 10^-6 to 10^-7 torr. These vapours the ionization chamber. If the sample is a gas or a volatile liquid then it can be injected directly.

 VAPOURIZATION:

 

 (ii)      IONIZATION CHAMBER:

            Vaporised sample of an bombarded with high-energy electron with the help of electron gun. When energy electrons collides with the neutral vapour phase atoms of the element, the collision provides the sufficient energy to remove an electron from the atom, forming charged positive ion.

           

            In case of molecules of compounds, the ions formed may be due to uni-positive molecule or due to break up of molecule. Such charged portions of molecule are called molecular fragments. The largest ion is the molecular ion. The fragments formed gives useful clues about the structure of the molecule.