This International Standard specifies two methods for the determination of mercury in drinking, surface, ground, rain and waste water after appropriate pre-digestion. For the first method (described in Clause 6), an enrichment step by amalgamation of the Hg on, for example, a gold/platinum adsorber is used. For the method given in Clause 7, the enrichment step is omitted. The choice of method depends on the equipment available, the matrix and the concentration range of interest. Both methods are suitable for the determination of mercury in water. The method with enrichment (see Clause 6) commonly has a practical working range from 0,01 µg/l to 1 µg/l. The mean limit of quantification (LOQ) reported by the participants of the validation trial (see Annex A) was 0,008 µg/l. This information on the LOQ gives the user of this International Standard an orientation and does not replace the estimation of performance data based on laboratory-specific data. It has to be considered that it is possible to achieve lower LOQs with specific instrumentation (e.g. single mercury analysers). The method without enrichment (in Clause 7) commonly has a practical working range starting at 0,05 µg/l. The LOQ reported by the participants of the validation trial (see Annex A) was 0,024 µg/l. It is up to the user, based on the specific application, to decide whether higher concentrations are determined by omitting the enrichment step and/or by diluting the sample(s). The sensitivity of both methods is dependent on the selected operating conditions. Another possibility for the determination of extremely low Hg concentrations down to 0,002 µg/l without pre-concentration is the application of atomic fluorescence spectrometry (see ISO 17852). Specific atomicabsorption mercury analysers allow determinations down to 0,010 µg/l without pre-concentration. In general, the determination of trace concentrations of Hg by AAS (or AFS) is dependent on clean operating conditions in the laboratory and on the use of high-purity chemicals with negligible low-Hg blanks. NOTE This International Standard may be applied to industrial and municipal waste water after an additional digestion step performed under appropriate conditions and after suitable method validation (see 7.4). A potential sample stability issue (mercury loss) for anaerobic reducing industrial effluents has to be considered thoroughly.