[el13]

There is an analytical  method to determine erucamide content of polyethylene similar to Kjeldhal method.Very short,the polimer is dissolved in benzilic alcohol ,NaOH conc(solid)is added the mixture is boiling one hour and a stream of nitrogen is taking ammonia in acid boric solution.For polyeyhylene samples the method is very good.I have tried this method for soil samples and the result was always bigger then the classic method TKN
I want to know your opinion.I am not a graduated chemist,I am only a worker in a petrochemical lab and I have a poor english.
Thank you

[Mr Peanut]

I don't mean to answer your question with another question but:

After trapping,  how are you determining NH3? Nessler? Selective Electrode?

[el13]

 Thank you very much for answer.
The same sample of the soil was also analized with Analytik Jena multi N/C 3100.
I am almost sure ,my method works very well but I do not know what are the compounds of nitrogen the method can find .
Elena.

[el13]

Reply again.I did not understand the word "trapping",now I can answer more correct: boric acid is used as the receiving solution.The boric acid captures the ammonia gas, forming an ammonium-borate complex.

[Mr Peanut]

Most analytical tests have two parts:
1: a preparative step(s)
2: a determinative step

When you analyze your polyethylene, your preparative step is distillation/trapping. Your determinative step is NDIR.

Now, the question is, are you preparing your sample for the soil NDIR analysis by conducting a TKN digestion followed by a distillation/trapping step (then NDIR) or are you using a solids analysis upgrade on your NDIR to determine total nitrogen?

If you are analyzing total nitrogen on your soil using the NDIR/solids-analysis approach then you will get higher numbers depending on the levels of nitrate and nitrite in the sample. TKN does not measure these varieties of nitrogen.

If you are using the TKN digestion step followed by distillation/trapping then NDIR, then the numbers should be similar. The best way to evaluate your methods for the species of nitrogen that TKN measures it to spike your samples with known amounts of an organic nitrogen compound (glutamic acid is commonly used) then measure % recovery.

Hope that helps.

[Mr Peanut]

Let me know if you are still unsure

[el13]

When I analyze my polyethylene I do not use the step of distillation.Actually this is the reason for I want to know if my method is good.
The sample is refluxing at boiling point of benzilic alcohol and with NaOH pellets ,in nitrogen atmosphere,and the ammonia gas is carry on by a stream of nitrogen ,trapped in a boric acid solution and titrated with sulfuric acid.
This method does not include neither digestion with sulfuric acid nor distillation .Digestion and distillation are the two reasons for I do not like classic TKN method.

[Mr Peanut]

I think it is time to run a glutamic acid spiked soil sample through both procedures.