Page 18 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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used for the lowest energy level, which is referred to as the ground state and therefore all practical
absorption measurements originate from atoms in the ground state, as do virtually all practical
fluorescence measurements. E and E refer to other energy levels, E being higher (greater energy) than
i
j
j
E. A solid vertical line refers to a transition involving the absorption or emission of radiation as energy.
i
The wavy line refers to a non-radiative transition.
The energy of the radiation absorbed or emitted is quantized according to Planck's equation (Eqn. 1.1).
These quanta are known as photons, the energy of which is proportional to the frequency of the
radiation.

where h = Planck's constant (6.62 x 10 J s), v = frequency, c = velocity of light (3 x 10 m s ) and l =
8
-1
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wavelength (m).
1.1.2 Mass Spectrometry

After the existence of different isotopic forms of the elements was first demonstrated by Thompson,
the technique of mass spectrometry is considered to have begun with Aston and Dempster, who
reported the accurate measurements of ionic masses and abundances in 1918-19. From these
beginnings, using magnetic and electric fields to separate ions of different mass, mass spectrometry has
grown into a major technique for the analysis of organic and inorganic compounds and elements. The
development of instruments based on quadrupole, time-of-flight, and ion trap mass analysers has taken
the technique from the research laboratory into everyday use as an analytical instrument.

In magnetic/electric sector mass analysers, ions are deflected in a magnetic field, the extent of
deflection depending on their mass-to-charge ratio (m/z). If all the ions have the same charge then the
deflection is dependent on their relative masses, hence a separation can be effected. In quadrupole
mass analysers the ions are subjected to a radiofrequency (RF) field, which is controlled so that only
one particular m/z can pass through it. Hence the quadrupole acts like a mass filter, and the field can be
varied so that ions of consecutively higher m/z pass through sequentially. An ion trap acts in a similar
way except that the ions are first trapped inside it, then let out sequentially. Time-of-flight mass
analysers are essentially long tubes along which the ions pass. Ions of low mass have a higher velocity
than ions of higher mass so, if a pulse of ions is introduced into one end of the tube, the light ions will
arrive at the other end before the heavy ions, thereby effecting a separation.