I would like to store rainwater samples for a long period of time (~1 month) prior to analysis for SVOCs (PAH, PCBs) and VOCs (bromoform, trihalomethane). So, what will be the best technique to store and preserve my samples? Should I freeze it or just refrigerate it at 4 degree celsius? Your recommendation is very much appreciated.
Hi Chin Yik Lin,
please have a look at the "MANUAL FOR THE GAW PRECIPITATION CHEMISTRY
PROGRAMME"
http://qasac-americas.org/sites/default/files/GAW-Precip%20Chem%20Manual%20No.160.pdf
The manual includes "Guidelines, Data Quality Objectives and Standard Operating Procedures".
Best regards
Uwe
Dear Dr. Uwe,
Thank you very much for the useful reference. From the manual, it is recommended that we should refrigerate our samples. I can follow the point. But, I am still looking for answers on freezing samples (for storage purposes). Can I freeze the samples? What is the effect of frozen samples on VOCs and SVOCs? If it is not recommended, why? Thank you very much prof.
Best regards,
Lin
Hi Lin,
chill samples to 4°C or below without freezing. Melting after freezing can result in degassing and VOC loss.
You can also make a test. Analyse some of your samples before and after you have frozen it for a couple of days and look if the results are different.
Best regards,
Uwe
Yes. I will try the test. Thank you very much for your enlightening ideas!
You can and should freeze the water samples in -18 centigrade, to keep them for long time. It works. I agree the previous suggestion for taking some tests to fresh and frozen sample. Usually it worked for me. DO not melt rapidly the samples.
You can not freeze your water samples for THMs (chloroform, BDCM, CDBM, bromoform). Your results will be highly unreliable. The samples will degase and lose the analytes. Water samples for THMs are typically collected in specially cleaned amber bottles with no headspace, shipped in cooled containers (as close to 4 C as possible) and analysed as soon as possible. They can not be preserved. If the samples are of drinking water, they typically contain the disinfection agent (chlorine, chloramines...) and formation of THMs continues in them after sampling, if you do not stabilize the samples, leading to artificially elevated results dependent on the time between collection and analysis. For drinking water samples, a quenching agent (thiosulfate, ammonium chloride or ascorbic acid) and a buffer (to bring the pH to under 4) are added to the sample at the time of collection, samples are collected without headspace, shipped and kept cold until analysis. We have established a maximum holding time of 14 days (using ascorbic acid) for such samples in our lab. Inadequate quenching and pH correction can lead to serious artifacts. Look for papers by Stuart Krasner if you are interested in THM samples stability.
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