Analysis of samples in situ and their preservation

Some indicators or components of water should be determined directly at the sampling point so that the errors due to sampling and transport of the samples to analytical laboratory are eliminated. Commercial instruments exist which enable one to determine an increasing number of indicators in situ. The first indicator values analysed are odour (sometimes also taste). 

If some of unstable components of water are to be determined, and if it is impossible to perform this analysis immediately on the spot or at the latest 12 hours after sampling, it is necessary to preserve the sample, in order to keep the components and their properties as they were at the time of sampling. However, preserved samples should be analysed at the latest on the third day after their sampling. The changes which appear due to the biochemical processes, can be partly eliminated or reduced by cooling the samples to 3-4°C. 

Preservatives and preservation methods used with water samples to minimize sample deterioration are summarized in Table 3.69 [8]. 

Preservative or technique used Effect on sample Type of samples for which the method is employed 

Nitric acid Keeps metals in solution Metal-containing samples 

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