Recently, I run the LCMS to analyze lipids (glycerol with 2 fatty acid chain, and 1 hydrophilic tail) with C18 column. I realized after running several times, the lipids are really stubborn, it sticks to the column. May I know what types of solvent can i use to remove this? I tried ACN, IPA (isocratic) but it seems not effective. Should I combine them?