According to Standard Methods for the Examination of Water and Wastewater, composite samples Keep samples at or below 4°C during compositing. Limit compositing period to 24 h. Use the same criteria as for storage of grab samples, starting the measurement of holding time from end of compositing period. State storage time and conditions as part of the results.
Some analytical compounds require chemical conservation. During a composite sample to analyse many compounds, the addiction of conservants should happen in the end of the composite sample, even collecting for 24 hours? Or this conservants should be added in each sample from the beginning of the sampling process?
If you intend to sample by putting the bottle under water, add preservative after taking the sample. If you are using a pump, tubing to sample water like with a programmable sampler, you can add the preservative at the beginning. If you are sampling a small stream where the water flows over rocks or a boulder or through a culvert, and abruptly drops, you may be able to sample with preservative in bottle, just don't let it overflow. If you only need small samples, and plastic bags are suitable, such as for fecal coliform tests in stream, Whirlpaks seemed to work well, but easiest to write on bag before taking sample. Depending on what is being analyzed, different preservatives may be needed. If you cannot preserve in the field sampling for some reason, then it needs to be done as soon as possible, and you may want to discuss to determine risk of sample changing, or try a few tests yourself, preservative in bottle, preservative after sample, transporting to lab and adding preservative and no preservative. Sometimes preservative may be a certain chemical, and sometimes acidification. Cooling on ice is always a good idea, but sometimes that also becomes very difficult.
4°C storage means preserving the sample and hence no alteration is expected unless and otherwise mentioned special situations like microbial sampling etc. Hence I think it doesn't matter adding preservative at the beginning or end of storage period. But in the case of microbial analysis temperature may affect the situation.
Hi Marcos,
Yes generally samples are preserved at 4°C. According to me the sample should be autoclaved as there might be microbial growth in it. These microbes might degrade chemical entities in the sample. I have not used conservants but that might help too.
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