Sample Preparation for Water Samples

Water samples can be acid digested to determine total metal content, using procedures as described above. Trace metals can be determined in this way because the concentrations are brought to a sufficiently high level when the 

The separation of waterborne metals into filterable and nonfilterable categories may be done if desired. This requires filtration of the water sample as soon after collection as possible, and certainly before any acid is added to the sample. The metals in natural water samples are often sorbed on particulate matter in a larger quantity than is present in solution. The particles may be filtered out and analyzed separately by digesting the filter in acid. On the other hand, if the total metal content of the water sample is required, the entire unfiltered sample is acidified and digested. 

An example of a method suitable for the determination of cadmium, cobalt, copper, iron, manganese, nickel, and zinc in water, using chelation and sample extraction, is as follows [113]. The sample is filtered through an acid-washed membrane filter as soon as possible after collection. It is then acidified with nitric acid for preservation until analysis. This will give the soluble metal fraction. If the total metal content is to be found, the sample is acidified and allowed to stand for 4 days with occasional shaking. Then it is filtered. 

The filtered sample is neutralized with ammonia, and then buffered sodium diethyldithiocarbamate (SDDC) is added. The pH is adjusted to approximately 6, and the sample, in a separatory funnel, is shaken thoroughly. The analyte is then extracted twice with organic solvent. Nitric acid is added to the solvent, and it is evaporated to dryness on a hotplate. The residue is taken up in nitric and hydrochloric acids, and the dissolved residue is analyzed by AAS. It should be noted that the soluble metals are those that pass through the 0.45-pm filter, while total metals do not include those that are so tightly bound into the particles filtered out that they were not solubilized in the slow, mild acid leaching process to which the sample was exposed. For a true total metal analysis, an acid digestion would be required. 

These methods must all be tested carefully, as the presence of a chelating agent, solid or dissolved, can shift the equilibrium between sorbed, complexed, and free ions in the sample. Metals in water samples can exist in several different forms. They can be sorbed on filterable particles, complexed 

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