Principle of mass Spectrometer

Determination of Atomic masses by mass spectrometry

An instrument that measures the exact masses of different isotopes of an element is called a mass spectrometer and this technique is called mass spectrometry.

First of all Aston’s mass spectrometer was used to identify the isotopes of an element. The Dempster’s mass spectrometer is used to identify isotopes of solid elements.

The principle of the mass spectrometer is that the positive ion of each isotope has its own (m/e) value. The isotopes are separated based on their mass-to-charge ratio (m/e).

Construction and Working:

The substance (isotopic element) under analysis is converted into vapuors and is kept very low (10^-6 to 10-7 torr). These vapours enter an ionization chamber. Here fast-moving electrons collide with vapours. Thus vapours are ionized. Different isotopes produce different ions with different (m/e) values. These positive ions are accelerated by applying a potential difference of 500-200 volts to the perforated negative grid. Then positive ions are passed through a strong magnetic field. This field separates the ions based on their (m/e) values. A simple diagram of the mass spectrometer is shown below.

After passing through a magnetic field the ions move in a circular path. Finally, these ions fall on an electrometer (ion collector) in the form of groups. The (m/e) of ions is determined by the following formula.

Formula: m/e = H²r²/2E

where “H” is the strength of the magnetic field, “E” is the strength of the electric field, and “r” is the radius of the circular path. The electrometer produces an electric current. The strength of electric current gives a relative abundance of ions with a definite (m/e) value. Thus the relative abundance of the isotopes is determined. This experiment is repeated for C-12 isotopes. By comparing two currents we can measure the exact mass number of isotopes. In a modern spectrograph, the electric current is amplified and fed to the recorder. The recorder makes a graph between relative abundance and the mass number of isotopes.