Sunday, 27 November 2016


Determination of Relative Atomic Masses of Isotopes 
By Mass Spectrometer

Mass Spectrometer
     Mass spectrometer is an instrument which is used to measure the exact masses of different isotopes of an element along with their relative abundance.
Determination of Relative Atomic Masses of Isotopes
     Different steps involved in the determination of exact atomic masses and the relative abundances of different isotopes of an element by Dempster's mass spectrometer are given below;
(i) Vapourization: The substance whose analysis for the separation of isotopes is required is converted into vapour state. That pressure of these vapours is kept very low, that is, 10⃜ ⁶ to 10⃜ ⁷ torr.
(ii) Ionization: These vapours are then allowed to enter the ionization chamber where fast moving electrons are thrown upon them. The atoms of isotopic element present in the form of vapours, are ionized. These positively charged ions of isotopes of an element have different masses depending upon the nature of the isotopes present in them.
(iii) Acceleration: The positive ions of each isotope has its own (m/e) value. When a potential difference (E) of 500-2000 volts is applied between perforated accelerating plates, then these positive ions are strongly attracted towards the negative plate. In this way, the ions are accelerated.
(iv) Deflection: The beam of accelerated positive ions is then allowed to pass through a strong magnetic field of strength H. This magnetic field is applied in a direction which is perpendicular to the path of the positive ions. The applied magnetic field helps in the separation of positive ions on the basis of their m/e values. The magnetic field makes the ions to move in a circular path. The ions of definite m/e value move in the form of groups one after the other and fall on electrometer.
(v) Mathematical Explanation: The mathematical relationship between m/e values and deflection in the circular path is
m/e = H²r/E
Where H = strength of magnetic field, E = strength of electric field, r = radius of circular path
     If E is increased, by keeping H constant then radius will increase and positive ion of a particular m/e will fall at a different place as compared to the first place. This can also be done by changing the magnetic field. Smaller the m/e of an isotope, smaller the radius of curvature produced by the magnetic field according to the above equation. Each ion sets up a minute electrical current.
(vi) Electrometer (Ion Collector): Electrometer develops electrical current. The strength of the current measured gives the relative abundance of ions of a definite m/e value.
(vii) Comparison with C-12: The ions of other isotopes having different masses are made to fall on the collector and the current strength is measured. The same experiment is performed with C-12 isotope and the current strength is compared. This comparison allows us to measure the exact mass number of the isotopes.
     In modern spectrographs, each ion strikes a detector, the ionic current is amplified and is fed to the recorder. The recorder makes a graph showing the relative abundance of isotopes plotted against the mass number.

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