Introduction - Determination of Anions in Natural and Treated Waters - page 14

Surface and ground
waters:

sulphate.

Potable water: chromate, dichromate, fluoride and sulphate.

Waste waters: chromate, dichromate, phosphate, silicate, free cyanide and total

cyanide.

Sewage effluents: free cyanide.

Trade effluents: free cyanide, silicate, sulphate and thiosulphate.

Since shortly after its inception in 1955, atomic absorption spectrometry has been the
standard tool employed by analysts for the determination of trace levels of metals in
water samples. In this technique a fine spray of the analyte is passed into a suitable flame,
frequently oxygen acetylene or nitrous oxide acetylene, which converts the elements to
an atomic vapour. Through this vapour radiation is passed at the right wavelength to
excite the ground state atoms to the first excited electronic level. The amount of radiation
absorbed can then be measured and directly related to the atom concentration: a hollow
cathode lamp is used to emit light with the characteristic narrow line spectrum of the
analyte element. The detection system consists of a monochromator (to reject other lines
produced by the lamp and background flame radiation) and a photomultiplier. Another
key feature of the technique involves modulation of the source radiation so that it can be
detected against the strong flame and sample emission radiation.
A limitation of this technique is its lack of sensitivity compared to that available by
other techniques (eg inductively coupled plasma atomic emission spectrometry).
Suitable instrumentation is listed in Table 1.3.

1.1.5.2 Inductively coupled plasma atomic emission spectrometry

This technique has, in recent years, been found to be particularly useful for the
determination in water of extremely low levels of a limited number of anions.

Non saline waters: bromide, chloride, fluoride, iodate, iodide, nitrate, nitrite and

tungstate.

Seawater: sulphate.

Surface and ground
water:

sulphide.

An inductively coupled plasma is formed by coupling the energy from a radiofrequency
(1-3kW or 27-50MHz) magnetic field to free electrons in a suitable gas. The magnetic
field is produced by a two- or three-turn water-cooled coil and the electrons are
accelerated in circular paths around the magnetic field lines that run axially through the
coil. The initial electron 'seeding' is produced by a spark discharge but, once the
electrons reach the ionisation potential of the support gas, further ionisation occurs and a
stable plasma is formed.