Dionex ICS1100 - rentention time and reproducibility issues

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Hi everyone,

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 !
It is an interesting problem but most likely an artefact. I guess that you are using an eluent generator and pure conductivity water. With that said, you did not mention how you are doing quantitation and the injection is manual or not? There are so many possibilities in your situation.

0. Do you control evaporation of the sample/ standard during the whole day? Since it is a trace analysis, the rate of evaporation will have significant effect in any part of the system. You may also try running blanks during different times of the day and see if carry over combined with evaporation is the cause of this apparent increase in the concentration. If everything is fine the carryover, if any, should remain the same (i.e blank injection should either show no peaks or very little peaks of the analytes).

1. Increase in retention time of fluoride and chloride: The retention of fluoride will increase slowly if you have metallic impurities collecting in the column or the suppressor, however, chloride may remain unaffected.

2. Do you also notice an increasing drift in the water dip as well and if the shape of the water dip degrades with time as well? If that is the case, it may be worthwhile to change the suppressor, to see if suppressor is the cause of the drift.
M. Farooq Wahab
mwahab@ualberta.ca
2 posts Page 1 of 1

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