Sample preservation prior to analysis - Determination of Anions in Natural and Treated Waters - page 771

Chapter 18
Sample preservation prior to analysis

18.1 General discussion

Aqueous solution of some anions eg chloride and sulphate are quite stable (ie do not
undergo decomposition) when stored up to one month at ambient temperatures. However,
it is necessary to consider what happens if the sample also contains a further anion which
upon storage decomposes to produce additional amounts of the anion of interest. For
example, if a sample which is required to analyse for sulphate also contains sulphide,
polysulphides, sulphite, thiosulphate, dithionate, polythionates then it is certain that the
true initial sulphate content of the sample at the moment of sampling will not be obtained
even if the sulphate analysis is carried out within 1h of the sample being taken. A similar
situation would exist for a sample in which it is required to determine chloride but which
also contains anions such as chlorite, chlorate and hypochlorite and possibly perchlorate.
In such circumstances the best that can be done is to analyse the sample by ion
chromatography for the anions of interest and all other likely interfering anions within 1h
of the sample being taken.
Williams [1] for example, has published an ion chromatographic method capable of
resolving for different sulphur containing anions which can be detected by placing an
ultraviolet and electrical conductivity detector in series after the elution column.
The retention times are as follows:

Retention time min. Detection

Sulphide 2.4 UV 205nm

Sulphite 3.8 EC

Thiosulphate 12.6 UV 195nm

Sulphate 22.8 EC

Similarly, the following chlorine-containing anions could be determined in a single run:

Retention time min. Detection

Chloride 4.6 UV 192nm

Chlorite 3.0 UV 195nm

Chlorate 15.2 UV 195nm

Perchlorate 19.6 EC