To remove polyimide outer coating from capillaries such as Simplus™ brand or Zero Flow™ or Controlled Flow™ capillaries for CE without using heat, it is best to use fuming sulfuric acid at 100°C.
You may want to heat the acid on a hot plate in a small shallow plate, using a fume hood and all the safety equipment needed. Then dip the capillary into the hot liquid. After a brief amount of time in the liquid, the polyimide should be able to be wiped away with a clean, soft cloth. Be careful not to apply too much pressure to the capillary. Remember that the detection window is fragile and can break easily.
Best practices in this case may call for practicing with capillary parts before you attempt on a coated capillary.
Click HERE for ordering information and pictures of CE Capillaries.