Declining peak response in ECD can be due to a number of problems including:
1). An electroactive component in your mobile phase. This could be due to a contaminant or in some cases solvents are intentionally fortified with such components. For example stabilized THF contains BHT which is electroactive and can result in the electrode fouling. A check for fouling is to polish electrode and see if this restores response. If so, you most likely have an electrode fouling problem. Of course, if you have a coulometric flowcell you won't be able to polish electrode and in this case I would suggest you consult the manufacturer's recommendation for cleaning contaminated electrodes
2). You have an electroactive "fouling reagent" in your sample. This is perhaps unlikely in your case since I presume you are only injecting standards when you observe the problem. See comments above for an electrode fouling test.
3). You haven't selected the right detection potential or are using the wrong waveform or the wrong reference electrode. What was the basis of selecting your operating conditions? Are you sure that these conditions have provided reliable results on an equivalent instrument?
