Page 95 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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Q. What determines the likelihood of spontaneous emission from an excited state?

Q. Which law describes the population of excited electronic states?

Q. Why are atomic emission lines not actually 'lines' in practice?

4.2 Excitation Sources.

4.2.1 Flame Sources

In the past, flames used for atomic absorption spectrometry have also been used for atomic emission
spectrometry, and these are described in some detail in Chapter 2. However, the advent of plasma
excitation sources has resulted in the demise of flame atomic emission spectrometry, for the reasons
discussed in Section 4.2.3.

4.2.2 Plasma Sources

A plasma is defined in the Oxford English Dictionary as

. . . a gas of positive ions and free electrons with an approximately equal positive and negative charge.
From the perspective of the atomic spectroscopist, desirable properties of plasmas include high thermal
temperature and sufficient energy to excite and ionize atoms which are purposefully introduced for the
purposes of analysis. In terms of atomic spectrometry, this means that we would generally wish to
measure the absorption or emission of radiation in the near-ultraviolet (180-350 nm) and visible (350-
770 nm) parts of the spectrum. In this sense, plasmas have been variously described as 'electrical
flames' or 'partially ionized gases'. A working definition for atomic spectrometry could be as follows:

A plasma is a partially ionized gas with sufficiently high temperature to atomize, ionize and excite most of the
elements in the Periodic Table.

A variety of gases such as argon, helium and air can be used to form analytically useful plasmas.
However, argon and helium have been used most extensively because they can be obtained in a
relatively pure form, have good characteristics for atomization, ionization and excitation of the analyte
and are readily available, although expensive, throughout most of the world.