I recently delved into hydrogeochemistry though it is not my area of expertise. I want to assess the level of toxic metals in an urban river plus the macrophytes and sediments. I would appreciate it if i could be enlightened on digestion procedures or methodology for water samples. Or is it the same way of digesting other biological matrices?
You can check the EPA METHOD 3052 MICROWAVE ASSISTED ACID DIGESTION OF SILICEOUS AND ORGANICALLY BASED MATRICES, METHOD 3050B ACID DIGESTION OF SEDIMENTS, SLUDGES, AND SOILS. Those may be useful even for biological samples. Just considering water samples, for some metals you will just need an acidification and then proceed with analysis. Other metals (like Ag) will need a kind of digestion.
Hope this helped.
It depends a bit on the metal, but in general terms it releases metals from their matrix and keeps them nicely dissolved.
Acid is not just used to digestion the matrix. It is also used to maintain the ions in solution. In acidic water, metal ions remain in solution, in basic water the ions may flocculate out of the solution, thereby influencing the overall concentration analysis of the solution.
You need to think about if you want to determine the content of toxic and heavy metal ions or any other ion in unfiltered or filtered water samples. What I normally would do is to filtrate the water sample with 0.45 µm membrane filter and acidify the sample with a small amount of pure conc. nitric acid. This steps are done a) to keep bacteria and sediment particles out and b) to stabilise metal ions in solution (as mentioned by Alan they might precipitate or stick to the walls of the container).
With sediments you would need to ask the question if you need the total metal ion content or only leachable or extractable content. Each of these questions will trigger long answers.... In any case the digestions methods of sediments are quite different to what is used for biological matrices.
Acidification of water samples (pH < 2) preserves most trace metals and reduces precipitation, microbial activity and sorption losses to container walls. As for the digetsion, Dr. Meisel got a point there. You need to take into consideration of your research objective before selecting suitable materials and method
Acidification depends on metal and on matrix. If a solid matrix is rich in silica, it is necessary to use fluoridric acid (in plastic flacons, and not glass flacons) in order to dissolve all the matrix and the metals which are potentially inside (be careful because fluoridric acid is very dangerous for the lab workers !).
In general, it is possible to acidify samples only arrived at the laboratory (in particular because heavy metals are not biodegradable and very few volatile) ,
I agree with the exception explained by Alan Kolok, but which can been involved only partiallly, for example if the sample is not filtered and if the ions flocculated are partially redissolved by the acid added to the sample at the laboratory).
Best regards
The answer probably depends upon whether or not you want to measure the "bioavailable" metal, or are more interested in total concentrations. Acid is very useful as a preservative, but could release previously non-toxic metal (i.e. not contributing to the effects observed in the organisms) which was bound by ligands or solid phases.
Nitric Acid to add based on the Trace element study. For many years the accepted protocol for the preservation of
water samples for determination of trace elements has required
adjustment of sample pH to less than 2 pH units using nitric acid
(U.S. Environmental Protection Agency, 1979). When using (Nitric Acid) a large volume of preservative or neutralizer, account for dilution by the acid
added to preserve the sample and/or the base used to adjust the pH to the range. Nitric acid is only easily preserve the sample when compare to the other acid. Because of The essence of the reactive process (Nitric Acid Formula) is to blend oxygen and ammonium nitrate (NH3) at high temperatures in the presence of platinum and rhodium (which act as catalysts in the process) to produce Nitrous Oxide which, when dissolved in water, produced nitric acid – also known as NHO3.
Acidification of water samples (pH < 2) preserves most
trace metals and reduces precipitation, microbial
activity and sorption losses to container walls. The acid
used (analytical grade, low metal content) must be
included in the blank(s) to be analysed in the
laboratory. For groundwaters and dissolved metals in
water samples, acidification should only be carried out
on filtered samples.
I think acidification of water sample with (1-2% nitric or HCl ) is very necessary and it must be with small amount (2:25) because for good solubility of metals found in the sample and to prevent accumulation and staking of the metals on the wall of the sample container and also to prevent the sample from contamination by fungi and other microorganisms.
digestion with Conc. HNO3 was always done to prevent percipitaion of metals, you should ad 5 ml of Conc. HNO3 to you collecion gar after collection and beofre storage. before colection you clean all you sampling materials with HNO3 AND DIW.
The metal generally get oxidized followed by precipitation and adsorption on the wall of container. To make in soluble form it is essential to add nitric acid as the metals nitrate are mostly soluble in water. Hence at the time of sampling acidification is necessary otherwise we get lesser quantity.
Together with above, it is used because it is known all metallic nitrates are highly soluble in water and nitric acid also the organic matter (dissolved and particulate forms) and kills the bacteria which can transfer the forms of the metals. Thus, acidification of water is needed and Nitric acid is very suitable as for all above reasons. If any other acids like sulphuric and hydrochloric are avoided because some mtallic chlorides and suphates are insoluble in water, PbSO4, PbCl2 etc.
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