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?