Engineering Chemistry S Datta

538 ENGINEERING CHEMISTRY
Schematic representation for production of a mass spectrum:
Injection of a small amt. of
sample into the instrument
Vapourisation
Sample
vapour
Bombarding with a beam of
high energy electrons
Deflection of moving stream
of ions with a magnetic field
to focus ions with a particular
mass on the detector.
Acceleration of the
positive ions in an
electric field
Production of molecular
ions M + .
Feeding the signal from the
detector to a computer which
prints out the mass spectrum
as the magnetic field steadily
changes over a range of values
to focus the series of ions with
different masses (m/z) one by
one in the detector.
80
% abundance
60
40
20
23 24 25 26 27
Mass of the positive ions detected
Fig. 24.22 Mass spectrum of magnesium.
The instrument needs calibration using a reference compound with a known structure
and molecular mass so that the computer can print a scale on the mass spectrum.
The mass spectrum for an element displays the relative abundance of different isotopes
of the element. This makes it possible to calculate the relative atomic mass for the element
(see Fig. 24.22).
Applications
• Structure elucidation
When a molecular compound is being analysed, the peak of the ion with highest mass is
usually the whole molecule ionised is known as ‘molecular ion’ or parent ion (M .+ ), which is the
relative mass (molecular weight) of the compound.