Fluoride matrix effects

Discussions about IC and related topics

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We have a Metrohm 882 running epa 300.0 and are having problems with quantifying to the expected concentrations of fluoride in certain matrices. Our QC samples (ICV, CCV, MS) consistently pass (+/-10%), but for some samples, we get ~ 50% expected recovery on certain wastewaters, and ~200% expected recovery on certain soils. Any ideas on what could be going on? We cleaned the suppressor with H2SO4/Oxalic acid but that didn't seem to help. MS/MSD of the problem samples get close to 100% recovery for fluoride which is what is confounding.
Dear Mritari

Samples containing alumina or titanium (as examples) may show low fluoride concentration by IC due to absorption. Only free fluoride is detected.
A too high response is more difficult to explain. Short chain organic acids might interfere.

Do not hesitate to send me chromatograms (*.idet) with details of the procedure (markus.laeubli@metrohm.com).
Dr. Markus Laeubli
Manager Marketing Support IC
Metrohm AG
9101 Herisau
Switzerland
I think Acetate can elute near Fluoride, depending on the column and eluent being used.
The past is there to guide us into the future, not to dwell in.
Acetate is very close and will co-elute. If the F level is high enough the acetate peak will add to the F peak and may not show up as even a bump in the peak. That might explain a high recovery, but not a low one. Running a high dilution of samples allows you to see if there is any co-eluting acetate. My F recovery on high level F decreases as my column ages. It only calibrates well up to 4 ppm when it's old. When new it works well up to 8 ppm. I would try dilutions.
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