Generally, it is recommended that ammonium and nitrate in KCI extracts should be analyzed within 24 h of extraction.  If this is not practically possible, extracts should be kept frozen.  In our lab we keep KCl extracts frozen at -18 °C until analyzed (usually up to about 3 months).  Once samples are thawed we complete the analysis within 2 days.  I am not sure how your results will be affected since you have not frozen the samples.  I suggest keeping these samples frozen until you are able to get them analyzed.

The book chapter available in the link below explains the methodology we follow in our lab.

http://www.cfs.nrcan.gc.ca/bookstore_pdfs/19121.pdf

Generally, it is recommended that ammonium and nitrate in KCI extracts should be analyzed within 24 h of extraction.  If this is not practically possible, extracts should be kept frozen.  In our lab we keep KCl extracts frozen at -18 °C until analyzed (usually up to about 3 months).  Once samples are thawed we complete the analysis within 2 days.  I am not sure how your results will be affected since you have not frozen the samples.  I suggest keeping these samples frozen until you are able to get them analyzed.

The book chapter available in the link below explains the methodology we follow in our lab.

http://www.cfs.nrcan.gc.ca/bookstore_pdfs/19121.pdf

Dear Mr. Sy Nguyen

We also follow the similar approach as described above by Dr. Jayasundara but frozen below -20°C for 2-3 months or even longer.    

Dear Mr. Sy Nguyen

I agree wtih both colleages,

we, nevertheless, find better(if the sample tipe allows it) to freeze the sample at -20°C up to 2 months, and later proceed with the extraction and measurement with in 24 h of extraction, rather than freeze the extract.

However, the best option is allways using fresh samples and inmediate extract mesurement  without freeze steps.

Dear Mr. Sy Nguyen,

I agree with the above, we usually freeze the extracted samples at - 20 C for a few weeks. And analysis it as soon as possible when it was thawed. If you just leave it in the fridge, there might be over oxidisation of NH4+.

All the best.

Thank you all, yes,actually, I frozen my samples at -20oC as well, and I am still feeling afraid of loosing NH4+ under this condition. From your experiences, I think my samples would be no big problems. Thank you very much indeed.

Dr Susantha Jayasundara and others have very correctly stated to freeze samples during storage. I will suggest ven then to analyze the samples at the earliest. Delay could alter the results. 

We analyze fresh soil extracts or within 24 hours.  Adding 0.8N sulfuric acid preserves the N balance for 24 hours when stored at 4 degree C in refrigerator. 

All this advice is great.  However, if you find that you cannot meet the schedule -- why don't you test retaining a number of different samples for the required time, and do the statistics on it? You may find you can accommodate storage of samples for a time. Compare it to freezing. Freezing your samples could also cause a problem If it results in accelerated transformations on defrosting. 

We used chloroform to fix water samples in the field (just make sure that chloroform does not interfere with the analytical device). Similar topic has been discussed earlier:

https://www.researchgate.net/post/Does_anyone_know_about_conserving_water_samples_prior_to_chemical_analysis

Notwithstanding the practical constraints in the analysis, any test has to be performed as per the specifications and standard procedure suggested by the author(s) of the analytical technique. We have no freedom or discretion to surpass the procedures based on personal issues and difficulties. These issues should have been predicted and addressed at the planning stage of experimentation itself. Or else, the modification adopted by a researcher should be specified in the research article or thesis of whatever when published. In this context, N values are bound to get vitiated on long term storage in a refrigerated conditions.

Hi Nguyen

I concur with the previous answers, I have found that when you keep the samples for more than 2 days, they give completely off results. It is to be noted though that my samples were not frozen, just kept them in the normal fridge.

The ionic activity gets slowed down at such low temperatures, but it doesn't come at a full stop, irrespective of the matrix the ions are in. Especially, with Nitrogen as central species in ions, it is quite more difficult for them to stay unreactive, since they have a reactive chemistry of ion form as well as an additional lone pair. The general trend tells us to keep them for not more than a month of refrigeration.

I add 0.1 ml (or less) of a highly concentrated solution of thimerosal (C9H9HgNaO2S)

(disolves up to 1000g/L) to a soil extract volume of approx 25 ml. This kills any microbial activity! The resulting systematical error towards lower concentration readings is smaller than the background noise of the analytical method itself. Be carefull: the stuff is highly toxic!

just found the detailed description in my files: 0.1 ml 0.61m Thimerosal per extraction sample (approx 10-10ml)

Freezing is good but isn't a panacea. Inorganic and small organic compounds turn over very rapidly in soil and (typical) soil extracts.  Hence, you really need to freeze the extract quickly (i.e. within an hour or less of extraction).  When you subsequently want to analyse the extract it needs to be defrosted, but as soon as the extract is above zero Celsius transformations and metabolism will recommence. 

One way of buying extra time would be to ultracentrifuge the extract through a MWCO filter so as to remove microbes and enzymes.  However, MWCO filters are expensive so I cannot imagine anyone doing this for routine analysis of lots of samples.   

Why are you wasting Mr.Toan, time which is irreversible in searching for options involving so many to circumvent self created problem? Your efforts are not on right path as far as the objective of research and for whose sake you are researching? Just for the degree? It is not worth, I tell you. Your object is similar to an effort to wake up the the frail and nearly dead. It is also similar to an effort to convert yoghurt in to milk.

Depend on the temperature of your fridge. If it is 4 C, no. If it is -20, -80 C, yes