Retention time problem

[ante1234]

Hi, I am new in chromatography and I need an advice. I am making method for detecting chloride, nitrate and sulfate in rain. System is Termoscientific ICS 5000+ capillary, column AS11HC, flow rate 0.015 ml/min, temperature 30 C, KOH eluent generator in isocratic mode at concentration 7.5 mM, instrument method time 17.5 minutes.
I done validation tests (accuracy, precision....) and everything seems ok, except there is constant decreasing in retention time, especially for sulfates, in period of 3 weeks from 12.5 minutes to 11.8 minutes and still decreasing. Retention time for chloride was 3.6, now is 3.5, nitrate was 7 minutes now ist 6.8.
Pressure/flow rate is stable, what can be a problem? Is it that maybe concentration of KOH eluent (7.5 mM) is too low for this method (column)?

best regards

[anionman]

Retention times always decrease as a column ages. I have had a couple of bad AS-18 columns that have had them shift pretty rapidly. I just had to recalibrate every other week and lower the eluent concentration to compensate. I have an Integrion with an AS-18 fast column and sulfate shifted earlier after a few months. The other anions are more stable. I compensated for that by increasing the column temperature. For some reason sulfate is more sensitive to temperature than the other anions on an AS-18 fast. I managed to get 18 months out of it by gradually moving the temperature from 30 - 35°.

[ante1234]

Dear anionman,
thank you for the answer and advice, for start I will also try to change a temperature and see if there will be positive effect on sulfate retention..

best regards