The analysis uses a polybutadiene/maleate copolymer on silica matrix column, eluted with 3mM methanesulfonic acid at 1ml/min, ambient temperature, non-suppressed conductivity detection. 4 ghost peaks are seen when water used to make up the mobile phase and samples is injected. The latter two match the retention times of sodium and potassium respectively, the former two elute earier but after the void. The phenomenon seems independant of the source of the water, and does not appear to be related to carry-over from the injector or syringe. The enquiry is on behalf of a colleague on the other side of the Pacific. Any ideas as to the cause of the problem and a possible solution are gratefully appreciated. Thank you for your time.

I don't know if this will help, but...
If you vary the injection volume, do your peak areas increase/decrease proportionaly? How about a "no injection" injection? Do you have a spare column to try?

You may be looking at a "system peak". These are very difficult to remove. Sometimes using ultra pure mobile phase constituents helps.

Were these "ghost peaks" positive or negative? If the potassium and sodium peaks were positive, the problem could be due to ion exchange retention in the sample loop due to contamination of the loop surface with biological organisms (a not uncommon problem when analyzing natural waters). If this is the problem, changing the volume of water passed through the loop prior to injection will result in a corresponding change in peak area. The solution in this case is to replace the sample loop. However, in some cases the injector rotor seal and associated fluid lines in the autosampler will also need to be replaced (if an autosampler is in use). With regard to positive peaks eluting prior to sodium, very few analytes elute before sodium (really only lithium which is a rare contaminant in deionized water), so the above mechanism is unlikely to be relevant for the peaks prior to sodium. Very likely, these are due to the system peaks as mentioned earlier. These peaks can be easily eliminated by the use of a suppressor. On the other hand, if these goes peaks are negative, the source of the problem is undoubtedly contamination of the methane sulfonic acid.