Water Analysis - Metals

Metals in waters can be analysed either on the filtered or unfiltered samples. If samples are filtered, then only the dissolved metals will be analysed by the ICP, but if they are not filtered, then any sediment may be included in the analysis. For the majority of samples, most contractors need the filtered (dissolved) metals, but if an effluent discharge is being monitored, it may be necessary to monitor the total loading into a water course, and this will include metals in the sediment.

It is very important for clients to specify if they require filtered (dissolved) or unfiltered metals when submitting water samples. Sometimes ‘total metals’ are requested, but this can mean ‘total dissolved’ to some clients, and is therefore ambiguous – it is preferable to use the terms filtered or unfiltered.

For best practice, water samples should be filtered on site through a 0.45 micron filter, but this does not always happen, due to time constraints for the site operatives. If samples are not filtered on site, it is possible for sediment to dissolve further into the sample, or with heavily contaminated waters, some salts may precipitate out of solution. Filtration with this size of filter will also remove bacteria, and so prevent further microbial degradation.

If samples are filtered on site, then preserved bottles can be used and these will ‘fix’ the analyte of interest to prevent further changes. If preserved bottles are used without filtering, then some sediment may dissolve in the preservative, leading to falsely high values.

Filtered waters are acidified with nitric acid prior to analysis by ICP-MS, and unfiltered waters are digested with a more aggressive mix of nitric and hydrochloric acids to dissolve any sediment, prior to analysis.
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Subpages (1): Solids in Water