Missing peak in GC-FPD

Hi there, I am using GC-FPD with a CP-Sil 5CB column (sulphur-specific) for sulphur-containing VOC detection. Initially, our purchased standard contains methyl mercaptan (CH3SH), dimethyl sulphide, carbon disulphide and dimethyl disulphide. I then further got standards for hydrogen sulphide (H2S) and sulphur dioxide (SO2). However, after a few injections of SO2 into the column, the CH3SH peak disappeared. I wondered if there was leaking in inlet or problem in liner so I checked leakage and replaced a brand new liner. But the CH3SH peak is still missing whereas the others are fine. I wonder was it the oxygens in SO2 contaminating the column or reacting with CH3SH. Could any experts comment on this or provide some suggestions on how I could clean the column? Or change the settings like changing purge flow/ time or hold time to help? 

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