Techniques of water sampling

In principle, water samples are to be taken such that 

the water samples taken, when analyzed in the laboratory, provide results which are representative of the water resource in question. 

Any secondary, physico-chemical, chemical or biological change to the water should be prevented. If such a change cannot be prevented, the changeable substances should be determined analytically on site, or fixed so that they can be recorded in the laboratory. Information should accompany the sample for the analyst conducting investigations. 

Water samples for determining phosphate may not be filled in bottles which have been rinsed with cleansing agents containing phosphate. The same applies to water samples to determine surface-active agents (detergents). 

Preservation of samples may be necessary in individual cases. The best form of preservation of samples is a rapid investigation of the water sample after sampling. This should take place in the laboratory as far as possible no later than 2 days after sampling. During transport and until commencement of the investigation the water samples should be stored in cool conditions at around C. 

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For liquid or solid samples more complex than water, a combination of techniques is commonly required. Certainly a first step involves a need to obtain the components of interest in a solution phase. This may either involve leaching of a solid or extraction of a liquid sample with or without concurrent concentration. If the components of interest are then obtained in a water system, the techniques applicable to water analyses are immediately available. Conversely, if the extraction is into a non-miscible organic solvent and the components sought can be reextracted into water by appropriate choice of pH, then again the techniques of water sample processing can be used. 

Extraction is the first step of pesticide residue analysis of water samples. Any pesticide residue technique of water samples should include the development of a dependable quantitative extraction procedure. The pitfalls of liquid-liquid extraction as now used are explained. A theoretical approach for quantitation of the extraction step based upon the thermodynamic partition coefficient as a p-value is proposed. The p-value approach is discussed to enable choosing the best solvent, water quality parameters, and solvent water ratios for serial analysis of water samples containing pesticides. 

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