[aqppaulet]

I am currently running Total N determination as well using a potassium persulfate digestion in an basic pH using an autoclave as a heat bath.All nitrogenous components are converted to NO3.
NO3 is reduced to nitrite in the presence of cadmium and mesure absorbance at 543nm against a distillated water blank.  

The problem I would like to ask about blank absorbances, there are often very high, about 0.600 - 0.700A. Is this a problem that is encountered frequently and could anyone offer any insights or suggestions that might help me?

Thank you;

[kgungor]

We are currently trying to validate a potassium persulfate digestion method for simultaneous determination of TN and TP in surface water samples.

We have experienced a similar problem (blanks having high absorbances). After some online research, we found that most of the lab grade potassium persulfate salts has significant N content. If this is the reason, you will need to obtain a low N (< 0.001%) potassium persulfate salt (e.g., Merck 105092) and recrystallize the salt to reduce its N content further. You can find the recrystallization procedures online.

[2stroke.hero]

I work in a lab that uses the persulfate TN TP digest and have a very similar problem!
Our TN blanks are always a little high (0.1 to 0.2 mg/l) as the persulfate does have some impurities, However sometimes we get a similar blank result *Ignore me, I am impatient* with the phosphate channel.  I don't think it's in the lab water, something interesting though is that the autoclave is used for the micro departments daily waste sterilization.... I have tested the waste water in the autoclave and it is very high in po4 and NOx/NH4.
I have ensured the digest tubes are sealed and have run some unsealed.  The results were the same??

I have been doing this test for 3 years and have still yet to identify whats going on?

I should say the SRP blanks with the same FIA, water etc are perfect.  This I confirmed on the ICP.

This is not some peak integration silliness, there are peaks well above the noise of the baseline?