Page 100 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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instruments. The simplest instrument is a flame photometer, which is frequently used for alkali metal
determinations in clinical and agricultural analysis. A low-temperature (e.g. air-natural gas) flame is
used; thus only the most prominent lines are excited. These lines are isolated by coloured glass or
interference filters (usually labelled K, Li, Na, etc.). Detection has typically been performed with a
*
barrier layer photocell , vacuum phototube, photomultiplier or solid-state detector.
In the past, much atomic emission work has been performed on atomic absorption instruments which
use a flame as the excitation source. However, these have been surpassed by instruments which utilise a
high-temperature plasma as the excitation source, owing to their high sensitivity and increased linear
dynamic range.

4.4 Inductively Coupled Plasma Atomic Emission Spectrometry

4.4.1 Plasma Generation

Many different plasma sources exist, but by far the most common is the inductively coupled plasma
(ICP). The ICP is generated by coupling the energy from a radiofrequency generator into a suitable gas
via a magnetic field which is induced through a two- or three-turn, water-cooled copper coil. The
radiofrequency energy is normally supplied at a frequency of 27.12 MHz, delivering forward power at
between 500 and 2000 W. Two gas flows, usually argon, flow in a tangential manner through the outer
tubes of a concentric, three-tube quartz torch which is placed axially in the copper coil (Fig. 4.5).
Because the outer and intermediate gases flow tangentially (i.e. they swirl around as they pass
through the torch), the plasma is continually revolving and has a 'weak spot' at the centre of its base,
through which the inner gas flow, containing the sample, can be introduced. When the gas is seeded
with electrons, usually by means of a spark, the electrons accelerate in the magnetic field and reach
energies sufficient to ionize gaseous atoms in the field. Subsequent collisions with other gaseous atoms
causes further ionization and so on, so that the plasma becomes self-sustaining. This occurs almost
instantaneously. The magnetic field causes the ions and electrons to flow in the horizontal plane of the
coil, thereby heating the neutral argon by collisional energy exchange, and a hot fireball is produced.
The hottest part of the ICP has a temperature between 8000
* A plate on which a thin layer of a semiconductor (e.g. selenium) has been deposited. A very thin transparent layer
of silver is sputtered over the selenium to act as a collector electrode. Light falling on the semiconductor surface
excites electrons which are released to the collector electrode. The current thus generated is measured using a
galvanometer.