How to clean a c-18 HPLC column

No Name,
If the frit is clogged, you can change it an hope not to damage the top of the bed, or try back flushing hoping not to create a void.
If your sample matrix fill the column with slow eluting compounds, try more and more non-polar eluatates. For example, start with your strong mobile phase, change to IPA for 5 or more column volumes, change to methylene chloride for the 5 volumes, and maybe even to hexane. Then change back down the solvents going from non-polar to polar.
If the sticking materials are polymers (like proteins), be careful not to set them in place. Use modifiers in the polar solvent that make the materials more soluble like salts, etc.
Jim

We often clean our column first 40 min with water, then 40 min with methanol, then THF, again methanol and next water. When you use a higher temperature this works better. Do not couple your column onto the detector, because if its really sticky material, its possible that the component blocks your detector cell, so you have to change this. Sometimes it helps when you clean and use your column in reversed way.

why do you wash for 40 min, not for 4 min?

Our suppliers suggest 15-20 column volumns, this is usually ±40 min @ 1.0 ml/min (in our case).