In this technique, a substance is first volatilized and then ionized with the help of a high energy beam of electrons. The gaseous positive ions, thus formed, are separated based on their masses to charge ratio (m/e) and then recorded in the forms of peaks. 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).
“Mass Spectrometer is an instrument which is used to measure the exact masses of different isotopes of elements.”
Types of Mass Spectrometer:
Aston’s Mass Spectrometer
First of all, Aston’s mass spectrograph was designed to identify the isotopes of an element based on their atomic masses.
Dempsters Mass Spectrometer
This was designed for the identification of elements that were available in the solid state.
Four Stages in Dempster Mass Spectrometer
There are four stages in Dempster’s mass spectrometer
The substance whose analysis for the separation of isotopes is required is converted into the vapour state. The pressure of these vapours is kept very low, that is, 10-6 to 10-7 torr. These vapours are allowed to enter the ionization chamber where fast-moving electrons are thrown upon them. The atoms of isotopic elements 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 plates. In this way, ions are accelerated.
These ions are then allowed to pass through a strong magnetic field of strength (H), which will separate them based on their m/e values. Actually, the magnetic field makes the ions 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. The mathematical relationship for (m/e) is
m/e = H2r / E
Where H is the strength of the magnetic field, E is the strength of the electrical field, r is the radius for the circular path. If E is increased, by keeping H constant then radius will increase and the 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. Each ion sets up a minute electrical current.
“Electrometer is also called as 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 experiment is performed with Carbon-12 isotope and the current strength is compared. This comparison allows us to measure the exact mass number of the isotope. The separation of Neon, Smaller the (m/e) of an isotope, Smaller the radius of curvature produced by the magnetic field according to the equation
m/e = H2r / E
In a modern spectrograph, 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.
Separation of Isotopes
The separation of isotopes can be done by the methods based on their properties. Some important methods are gaseous diffusion, thermal diffusion, distillation ultracentrifuge, electromagnetic separation and laser separation.