Determination of Relative Atomic mass of Isotopes by Mass Spectrometer

         Mass spectrometer is an instrument which is used to measure the exact masses of different isotopes of an element. 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. Actually, mass spectrum is the plot of data in such a way that (m/e) is plotted as abscissa (x-axis) and the relative number of ions as ordinate (y-axis).

Types:

          First of all, Aston's mass spectrograph was designed to identify the isotopes of an element on the basis of their atomic masses. There is another instrument called Dempster's mass spectrometer. This was designed for the identification of elements which were available in solid state.

Its work:

          The substance whose analysis for the seperation of isotopes is required, is converted into the vapour state. The pressure of these vapours is kept very low, that is, 10 power -6 to 10 power -7 torr. These vapours are 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.

      The positive ion 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.

      These ions are then allowed to pass through  a strong magnetic feild of strength (H), which will seperate them on the basis of their (m/e) values. The magnetic feild makes the ions to move in a circular path. The ions of definite m/e value will move in the form of groups one after the other and fall on the electrometer.

      Electrometer is also called an ion collector and develops the electrical current. The strength of the current thus measured gives the relative abundance of ions of a definite m/e value. 

      Similarly, the ions of other isotopes having different masses are made to fall on the collector and the current strength is measured. The current strength in each case gives the relative abundance of each of the isotopes. 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 isotope. Smaller the (m/e) of an isotope, smaller the radius of curvature produced by the magnetic feild.

       In modern spectrographs, each ion strikes a detector, the ionic current is amplified and is fed to the recorder.

Methods of isotopes seperation:

      The seperation of isotopes can be done by the methods based on their properties. Some important methods are, gaseous diffusion, thermal diffusion, distillation, ultracentifuge, electromagnetic seperation and laser separation