Introduction - Determination of Anions in Natural and Treated Waters - page 15

The neutral particles are heated indirectly by collisions with the charged particles upon
which the field acts. Macroscopically the process is equivalent to heating a conductor by
a radio-frequency field, the resistance to eddy-current flow producing joule heating. The
field does not penetrate the conductor uniformly and therefore the largest current flow is
at the periphery of the plasma. This is the so-called 'skin' effect and, coupled with a
suitable gas-flow geometry, it produces an annular or doughnut-shaped plasma.
Electrically, the coil and plasma form a transformer with the plasma acting as a one-turn
coil of finite resistance.
The properties of an inductively coupled plasma closely approach those of an ideal
source for the following reasons:

• The source must be able to accept a reasonable input flux of the sample and it should be
able to accommodate samples in the gas, liquid or solid phases.
• The introduction of the sample should not radically alter the internal energy generation
process or affect the coupling of energy to the source from external supplies.
• The source should be operable on commonly available gases and should be available at
a price that will give cost-effective analysis.
• The temperature and residence time of the sample within the source should be such that
all the sample material is converted to free atoms irrespective of its initial phase or
chemical composition; such a source should be suitable for atomic absorption or
atomic fluorescence spectrometry.
• If the source is to be used for emission spectrometry, then the temperature should be
sufficient to provide efficient excitation of a majority of elements in the periodic table.
• The continuum emission from the source should be of a low intensity to enable the
detection and measurement of weak spectral lines superimposed upon it.
• The sample should experience a uniform temperature field and the optical density of the
source should be low so that a linear relationship between the spectral line intensity
and the analyte concentration can be obtained over a wide concentration range.

Greenfield et al. [2] were the first to recognise the analytical potential of the annular
inductively coupled plasma. Wendt and Fassel [3] reported early experiments with a
'tear-drop'-shaped inductively coupled plasma

Table 1.3 Available flame and graphite furnace atomic absorption spectrometers

Type
instrument

Supplier Model no
and type

Microprocessor Hydride
and
mercury
attachment

Auto-
sampler

Wavelength
range

Flame
(direct
injection)

Thermo-
electron

1 L 157
single
channel
1 L 357
single

Yes Yes -