Fluorid peak is low

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We have a 883 Basic IC plus, on whitch we analyse the anions Fluoride, Chlorde, Nitrate and Sulfate. I resently change the analytical column and calibrated for all the anions. After this, all the controls were fine the first day of analysis. The next day, the fluoride peak became smaller and smaller for each time I ran the control. I tried to change the inline filters, pump tubings, and remove the guard column. After that the background conductivity beacme really high (above 600 µS/cm). I then cleaned the MSM, and the background conductivity became normal, but the Fluoride anion-peak/area is still too low.

In the internal controle we are using, all the anions mentioned are present, but only the fluoride peak is lower than normal. The others are within acceptabel values.

Have anyone had a similar problem, and can help me with what to do next?
Is the fluoride reaching a stable area or does it keep shrinking? Did the column come with a Quality Assurance Report? What was the area of F on that? It's also possible that you don't have stable flow. How old is the instrument? My Dionex IC goes through motors every 6-7 years. A few years ago the sulfate and nitrate peaks suddenly got too close and it was because of an inconsistent flow rate due to the motor going bad. The baseline of the blanks was fine.
Dear ikauster

Reduced fluoride response quite often is a result of non-sufficiant regeneration.
Please use min. 100 mmol/L sulfuric acid for peristaltic pump regeneration or 500 mmol/L for Dosino regeneration.

Best regards
Dr. Markus Laeubli
Manager Marketing Support IC
Metrohm AG
9101 Herisau
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