Introduction - Determination of Anions in Natural and Treated Waters - page 18

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but later described the medium power (1-3kW), 18mm annular plasma now favoured in
modern analytical instruments [4].
The current generation of inductively coupled plasma emission spectrometers provide
limits of detection in the range of 0.1-500µg L −1 in solution, a substantial degree of
freedom from interference and a capability for simultaneous multi-element determination
facilitated by a directly proportional response between the signal and the concentration of
the analyte over a range of about five orders of magnitude.
The most common method of introducing liquid samples into the inductively coupled
plasma is by using pneumatic nebulisation in which the liquid is dispensed into a fine
aerosol by the action of a high-velocity gas stream. To allow the correct penetration of
the central channel of the inductively coupled plasma by the sample aerosol, an injection
velocity of about 7m s − 1 is required. This is achieved using a gas injection with a flow
rate of about 0.5-11min −1 through an injector tube of 1.5-2.0mm internal diameter.
Given that the normal sample uptake is 1-2ml min −1 this is an insufficient quantity of gas
to produce efficient nebulisation and aerosol transport. Indeed, only 2% of the sample
reaches the plasma. The fine gas jets and liquid capillaries used in inductively coupled
plasma nebulisers may cause inconsistent operation and even blockage when solutions
containing high levels of dissolved solids, such as sea water or particulate matter, are
used. Such problems have led to the development of a new type of nebuliser, the most
successful being based on a principle originally described by Babington (US Patents). In
these, the liquid is pumped from a wide-bore tube and thence conducted to the nebulising
orifice by a V-shaped groove [5] or by the divergent wall of an over-expanded nozzle [6].
Such devices handle most liquids and even slurries without difficulty.
Nebulisation is inefficient and therefore not appropriate for very small liquid samples.
Introducing samples into the plasma in liquid form reduces the potential sensitivity
because the analyte flux is limited by the amount of solvent that the plasma will tolerate.
To circumvent these problems a variety of thermal and electrothermal vaporisation
devices have been investigated. Two basic approaches are in use. The first involves
indirect vaporisation of the sample in an electrothermal vaporiser, eg a carbon rod or tube
furnace or heated metal filament as commonly used in atomic absorption spectrometry
[7-9]. The second involves inserting the sample into the base of the inductively coupled
plasma on a carbon rod or metal filament support [10,11]. Available instrumentation is
reviewed in Table 1.4.
1.1.6 Polarographic and electrochemical methods
1.1.6.1 Polarography
This technique has been applied to the following 20 determinations in water, all of which
are capable of undergoing an oxidation reduction (ie redox).