Cation exchange chromatography peak shape issues

[rwk27834]

A client is having problems with peak shape, especially at lower concentrations, in cation exchange chromatography (CEC). Rather than describe it as tailing or fronting, it prefer to just call it "bad". Their main (among several) peaks of interest is ammonium ion, and it is the peak shape of that which I am most concerned about. They are using a Dionex CS-18 column, a concentrator column and a suppresor. Their eluant is dilute H2SO4 (~ 15 mM). Does anyone have ideas on improving peak shape in CEC? The water they use is Caledon Laboratory Grade (10 MegOhm). Is that insufficiently pure? Should the mobile phase (eluant) be buffered? I have a lot of experience with HPLC in general, but not in cation exchange chromatography.

[DR]

For IC, the water can never be too pure. From 10M-ohm, there is considerable room for improvement.

[Mark Tracy]

I asked our resident expert on the CS18 column. She says that if only ammonium is affected, then it is probably an instrument configuration issue. If you are using eluent recycling to regenerate the suppressor, then ammonia can diffuse back across the suppressor. One fix is to use an external source of water to regenerate the suppressor. Another is to use a long delay loop between the detector outlet and regenerant inlet.