The relative masses of isotopes and their relative abundance can be determined using a mass spectrometer. The mass spectrograph was invented by a British physicist, F.W. Aston, in 1919. It is an instrument that separates ions of different masses. The results are recorded on a photographic plate. The modem version records results electrically and is called mass spectrometer. Fig. 5.10 is a schematic diagram of a mass spectrometer.
Fig 5.10 A diagram of a mass spectrometer
5. Detection. The ions are detected by the detector and a record is made.
The substance to be analysed is vapourized by heating and introduced into ionization chamber. In the ionization chamber, atoms of the element are bombarded with a stream of high energy electrons. The high energy electrons knock out one or occasionally two electrons from the atoms resulting in the formation of positive ions.
The positive ions are accelerated by passing through an electric field. The accelerated beam of ions passes through the magnetic field where the ions are deflected according to their mass and their charge.
For a specific accelerating electric field and deflecting magnetic field, ions of only one particular mass/charge ratio hit the detector at the end of the apparatus. Ions of smaller mass/charge ratio will be deflected too much whereas ions of greater mass/charge ratio will be deflected too little. The ion detector is linked through an amplifier to a recorder. As the strength of the magnetic field is slowly increased ions of increasing mass will be detected and a mass spectrum as shown in Fig. 5.11 is recorded.
Fig. 5.11. A mass spectrum for chlo
The relative heights of the peaks in the mass spectrum gives in the relative abundance of different ions present. The mass spectrometer can be used to find the number of isotopes of an element, and also to determine the relative isotopic masses of various isotopes. From the knowledge of relative isotopic masses and their relative abundances, the relative atomic mass of an element can be calculated.
A naturally occurring element which has no isotope will show only one peak in mass spectrum.
DETERMINATION OF RELATIVE MOLECULAR MASS
Mass spectrometer also helps in determining the relative molecular mass and molecular formula of compounds, especially organic compounds. When the molecules of an organic compound pass through a mass spectrometer, they undergo fragmentation. As a result, the mass spectrum of the compound contains several peaks, each corresponding to a particular fragment. However, a few molecules may pass through intact. These intact molecules give a peak showing the relative molecular mass of the compound. For example, from the mass spectrum of an unknown compound, as given below, we find that the relative mass of the heaviest particle recorded is 58. We can assume that this corresponds to the intact molecule with single positive charge. Hence, the relative molecular mass of the compound must be 58. Knowing the empirical formula of the compound, its molecular formula can be determined.
Fig. 5.12. Mass spectrum of a compound
Example 5.5 If bromine atom is available in the form of, say, two isotopes (49.7%) and Br (50.3%), calculate the relative atomic mass of bromine.
Solution .% of Br-79 = 49.7%;% ofBr-81 = 50.3%
Relative atomic mass of Br = 79 x 49.7+ 81 x 50.3 / 100 = 80.0
Thus, relative atomic mass of bromine is 80.0.