problem for the anion determination in waste water

[Ericelabor]

Hi,
I have a problem with the determination of anions by ionic chromatography. I am working in waste water and the injection volume is 10 µl. I use Na2CO3 9 mM for eluent. The column is a IonPac AS 9-HC with a AG 9-HC. This column is recommended by Dionex and has a good separation. I use a 45 µm filter and I don’t mineralisate the samples. W e have a UV-detector for the nitrite determination.
I do inter-laboratory comparison to check the method.
For the phosphate all tests (at least 5 inter-laboratory tests) are unsuccessful ! In the last interlaboratory comparison we found 1.776 P-ppm and the other laboratory have an average of 2.523 P-ppm. In all tests we found below the expected concentration.
In the last test I have a problem with fluoride and nitrite. In the first case we found 3.5875 ppm and the average is 4.0761 ppm and for nitrite we found 4.1775 Nppm and the average is 3.4337 Nppm.
I have done a new calibration and I have a calibration from 0.2 ppm to 5 ppm for fluoride, 0.2 NO2-ppm to 50 NO2-ppm and 2 PO4-ppm to 50 PO4-ppm.
If I check the method with a certified material from LGC promochem I found the good value.
Thank you for your advice.

[H2Oh]

I can't say with certainty what your problem is, but I too am running a Dionex based anion analysis. I work primarily with industrial waters and find that iron interferes significantly with my phosphate readings. I suspect the iron strongly binds with the column preventing further phosphate interactions and then flushes out at seemingly random times which will sometimes liberate some phosphate which as been trapped in the column as well. This happens with both the AS17 and the AS22. I don't know the AS9 composition.

In this way, most standards appear ok as they don't have unwanted iron but samples can run low. I wish I had a good fix for you, but we run phosphate via a wet test to compensate. Do you have high iron in the wastewater?

Nitrite has never given me any trouble but I'm using conductivity rather than UV detection for it. We've calibrated 0.1 up to >100 ppm with strong linearity still. I have no experience with fluoride, sorry.

[Ericelabor]

Hi,

Thank you for your advice.
In fact I have no metals. We have done a ICP screening and the concentration is below 15 ppm in one metal. Iron is below 0.1 ppm !