Page 109 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon
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gas flow exiting it has sufficient velocity to punch a hole through the centre of the ICP, thereby forming
an annular or doughnut-shaped plasma. The aerosol is successively desolvated, decomposed and
atomized, and the atoms formed are subsequently ionized and excited in the ICP.
Q. On which principle is the operation of a pneumatic nebulizer based—explain how this works?
Q. If you wished to nebulize sea-water with a Meinhard nebulizer, what would you have to do first,
bearing in mind that sea-water contains 3.5% NaCl?
Q. Make a list of the advantages and disadvantages of the ultrasonic and Babington-type nebulizers.
4.4.4 Excitation.
Once atoms or ions are formed in the ICP, there are several pathways by which they become excited
and we usually talk in terms of excited-state atoms and ions. When an electron absorbs energy it is
excited to a higher energy state; as it falls back to the ground state it emits radiation which is
characteristic for that particular transition. If the electron absorbs sufficient energy, equal to the first
ionization energy, it escapes the hold of the nucleus completely and an ion is formed and another
electron can be excited. In the simplest case where TE or LTE holds, the number of atoms or ions in the
excited state is given by the Boltzmann distribution, described in Section 4.1. However, this only gives
an approximation of the populated states in the ICP because it is valid only under TE conditions.
Several different mechanisms have been proposed to explain the populations of the various excited
atomic and ionic states. The main mechanisms are as follows.
(i) Thermal excitation/ionization—caused by collisional energy exchange between atoms, ions and
electrons, e.g.
where the asterisk denotes an excited state.
(ii) Penning ionization/excitation—caused by collisions between ground-state atoms and argon
metastable species, e.g.
