If raw LPG is washed with 10% caustic solution, whether only H2S is removed or mercaptans/others too removed?

I feel to remove mercaptans from raw LPG is difficult to remove than H2S. Therefore while a 10% caustic wash is done H2S is removed & mercaptans stay there. In addition why we should remove mercaptans while we add ethyl mercaptan again in storage tanks?