The amount of trichloroammine platinate K[PtCl3(NH3)] in Cisplatin is determined with the HPLC method of the USP25. As mobile phase 0.8 ammonium sulfate in 2 L water at pH 5.9 is used and as stationary phase a strong anion exchange column.
As we first used the column the retention times were 1.8 min for cis and 6.4 for tri but rapidly decreased to 1.7 and 3.1 min and dropped even further. Obviously impurities bind to the column and / or the stationary phase leaks from the column. What do we have to do to stabilize the column?

wim

What type of column are you using? If the column is polymer based, it's unlikely that the eluent has caused any damage to the stationary phase. In the case of a silica based column, this pH should still be okay but of course such a stationary phase is considerably less stable at this pH than a polymer based material (which should last for years at this pH). More likely, your sample contains a highly retained impurity which is "fouling" stationary phase. If I understand your system, you are using 0.8 molar ammonium sulfate as the eluent. If so, this should be capable of eluting highly charged species but not necessarily highly polarizable species. You might want to try rinsing the column with a more potent eluent anion to remove the contaminant followed by regeneration with the running eluent. Relative elution strength for eluent anions: chloride, sulfate < nitrate < iodide < thiocyanate < perchlorate. The relative elution strength of chloride and sulfate is ionic strength dependent but at 0.8 molar they are relatively comparable. Of course, the higher the elution strength of the eluent anion, the longer it will take for sulfate to restore the column back to its original sulfate form.