Analytical Chemistry

What is Mass Spectrometry?

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A technique which is used to find out relative abundance of isotopes with respect to their atomic masses in the form of graph, is called Mass Spectrometry.

Is mass spectrometry quantitative or qualitative?

                Mass spectrometry is used for quantitative as well as qualitative analysis.

What is a mass spectrometer used for?

An instrument, which is used to separate positively charged particles on the basis of their m/e values and to get the record on the photographic plate or electrometer, is called mass spectrometer.

Parts of Mass Spectrometer:

Mass spectrometer consists of the following parts:

1. Vaporizer                                       2. Ionization chamber

3. Electric field                                   4. Magnetic field

5. Recorder

Determination of Relative Atomic Masses of Isotopes:

First of all, Aston’s mass spectrograph was designed to identify the isotopes of an element on the basis of their masses. Then Dempster’s mass spectrometer was designed to identify the elements available in the form of solids.

How does a mass spectrometer work?

  • 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 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 field of strength (H), which will separate them on the basis of their (m/e) values. Actually the magnetic field 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.

The mathematical relationship for (m/e) is

m/e = H2r2/2E


H= the strength of magnetic field

E = the strength of electrical field

r = the radius of circular path.

  • If E is increased, by keeping H constant then radius will increase and positive ion of 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.
  • Similarly, the ions of other isotopes having different masses are made to fall on the collector and the current strength is measured. This measurement of strength of current is used to find out relative abundance of isotopes.

Watch Video Lesson Here: Mass Spectrometry

What is the basic principle of mass spectrometry?

In modern techniques, each ion falls on a detector, the current produced by ions is amplified and finally send to the digital recorder. On the basis of results from ion detector, we get a graph between Mass Number & Relative abundance of isotopes.

The separation of isotopes can be done by the methods based on their properties.

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