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- Posts: 1
- Joined: Thu Oct 17, 2013 4:55 pm
Our lab has a Dionex ICS-1100 instrument, with an AS19 (4x250mm) colomn and an AG19 guard column, an ASRS300 (4mm) suppressor. We are trying to measure fluoride, chloride, bromide and iodide in the 2ppm range for F and Cl and 20ppb range for Br and I, using a KOH eluent at 15 mM.
We are facing reproducibility issues which are correlated to retention time issues.
If we measure a 5ppm fluoride and chloride standard solution over days, we see a concentration increase from, for example, 4.7ppm at the begining of the day and 5.3ppm at the end. Fluoride and Chloride evolution are very similar, but this evolution is not reproducible from days to days : for the same standard solution, and for the same instrument pressure (around 2200-2400 psi), some days concentration "re-starts" at 4.7ppm and some days it "continues" to increase from what it had been the day before.
At the end we have a 4% RSD for reproducibility, which is not as low as we wanted it to be.
At the same time, we noticed that retention times were in some way correlated with that increase : species tend to eluate later and later as the day goes, even though the retention time shift is very low (less than 1 min)
I would really appreciate any suggestions or explanations, thank you !