Preservation and storage of water samples

PRESERVATION AND STORAGE OF WATER SAMPLES FOR THE DETERMINATION OF ORGANIC COMPOUNDS... 

Sliwka-Kaszynska, M., A. Kot-Wasik, and J. Namiesnik. 2003. Preservation and storage of water samples. Crit. Rev. Environ. Sci. Technol. 33 31—14. 

Collection and handling of aqueous samples for low-level determination of mercury must address factors such as whether the sample is representative of the system sampled, possible interconversion processes, contamination, as well as preservation and storage of the sample before analysis. The measurement (sampling and analysis) protocol must be even more carefully designed if speciation of mercury forms in aqueous samples is intended. There have been remarkable improvements in sampling and analytical techniques that have resulted in a dramatic increase in the reliability of data for mercury levels in water samples over the past 15 years. The stability of mercury in solution is affected by many factors. These include (1) the concentration of mercury and its compounds, (2) the type of water sample, (3) the type of containers used, (4) the cleaning and pretreatment of the containers, and (5) the preservative added. 

Once the water samples are collected, they should be stored at 4°C until extraction, which is usually performed within 48 h. Storage of water samples for longer periods may lead to degradation of target compoimds in water samples [50]. For longer preservation of water samples, chemical agents such as formaldehyde (1% v/v) and sulfuric acid (pH < 3) can be added to the water samples to prevent bacterial activity. 

Unfiltered and unpreserved groundwater water samples collected for total and dissolved metal analyses arrived to the laboratory in a cooler with ice three days after collection. On the fourth day after collection the laboratory filtered the samples for dissolved metal analysis and preserved all samples with nitric acid. The violation of the preservation requirements (no acid ice instead of ambient storage temperature) had a marginal effect on the concentrations of total metals as the addition of acid dissolved most of the metals that may have precipitated in the sample container. That is why the chemist accepts the total metal results, but qualifies them as estimated data. However, because improper preservation and storage have grossly compromised dissolved metal concentrations, the chemist rejects the dissolved metal results and requests that the water be resampled and reanalyzed. 

Kotlash, A.R. and B.C. Chessman. 1998. Effects of water sample preservation and storage on nitrogen and phosphorus determinations Implications for the use of automated sampling equipment. Water Res. 32 3731-3737. 

Two procedures are normally used to preserve sample integrity as much as possible storage of the sample frozen at the temperature of -20°C or lower, and acidification of the sample by addition of mineral acids (pH < 2). The two procedures are about equivalent if total dissolved metal concentration needs to be determined, while acidification of the sample must be avoided if a speciation study is to be carried out because this procedure would certainly alter the complexation equilibria of metals in sea water. There is some experimental evidence that freezing of samples does not significantly alter the results of the complexation studies carried out on heavy metals according to the methodology discussed here (53). 

Ideally samples should be analyzed in situ or at the site of sampling in the field. If direct measurements are not possible or are too expensive, samples should be analyzed as soon as possible to avoid the need for preservation. However, samples cannot always be analyzed directly and they may have to be stored for so long that preservation is necessary. The preservation of water samples is covered in ISO standard 5667/3 (1985). The preservation methods described in this standard include time limits for sample storage and analysis specifications for container material prevention of exposure to light temperature control (2-5°C) pH control (addition of sulfuric, nitric, or phosphoric acid, or sodium hydroxide) addition of special reagents (e.g., ethylenediaminetetraacetic acid (EDTA), copper sulfate, zinc acetate, formaldehyde) to retard biological activity, hydrolysis of compounds and complexes and measures for reducing volatility of compounds and sorption effects. 

The role of filtration of water samples at the time of collection and in relation to storage and preservation of the sample is often an important consideration. Many substances of interest may be present in a water sample in particulate as well as soluble form. Filtration removes particulate matter, so that a decision on whether to filter at the point of collection will depend on the objectives of the study. Another consideration relevant to filtration and the possible presence of particulate matter are the effects on such matter of adding a sample preservative such as acid. Generally, it is sound practice to filter before adding a preservative that may solubilize particulate matter or leach contaminants from it. 

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