Does anybody know any technique of regenerating old C8 columns?

Depends on why they died.

In general, if the column packing itself is damaged (e.g., loss of bonded phase at low pH or dissolution of silica at high pH, there's very little hope.

If the column has developed a headspace, then it can be "topped off" with packing from another column. In my experience, though, this is only a short-term solution.

If the column is contaminated with strongly-bound garbage, then you need to find something that is a good solvent for your particular flavor of garbage (e.g., methylene chloride for lipids, iPrOH or guanidine HCl for proteins). Then simply go from your mobile phase to that solvent and back, using intermediate solvents as necessary to avoid immiscibility and/or precipitation. A typical sequence for lipids might be:
- mobile phase
- buffer-free mobile phase
- methanol or ACN
- MeCl2
- methanol or ACN
- mobile phase
It takes about 10 column volumes of solvent to get complete wash-out.

A void at the head of the column can be identified by running the column 'reversed' from the normal installation. Before testing the column, install the column backwards, don't connect any tubing after the column, and wash any particles off the column head. Then test it. Be careful you don't damage/plug the detector cell or tubing!

If there is a void, the reversed position will give you better resolution---for a while. At least you will know what the problem is.

If there is no change in the resolution, then the problem is more likely damage to the absorption mechanism--unremoved garbage, etc.

If a column has had a guard column on it, there shouldn't be any particles to worry about.

Once-upon-a-time we had problems with the guard columns themselves, and isolated the problem this way. Some guards would maintain resolution for many days, others only for a single day.

I typically reverse my guard-column protected columns every few days of running, and my analyses stay stable for a long time. I feel that guard columns, in addition to the usual claims, also protect the analytical column from high pressures caused by particle build-up.

In the early days of home-built post-column systems, we lost columns regularly from the NaOH dissolving the packing at the end of the column if the system accidentally shut down overnight. It also plugged the detector tubing when we re-started it, making tightly packed, useless micro-columns.

I've washed and tested many columns with unknown histories, and maybe half of them were useful for something; some were good as new!!

This is just my opinion--your mileage may vary!

One more thing. Before you spend a lot of time doing regeneration, consider the cost versus what that same amount of time spent working with the new column formats (2.1 x 150, for instance) will get you. Unless you are working with current formats and columns, it may be a waste of time and money. The 'micro' bore columns will pay for themselves in saved solvent, compared to old columns. Who would bother to install a 10um column these days?

We are close to junking our entire inventory of perfectly good 4.6 x 150 or 250 columns, now that we realize we won't wear them out for years. And we'll be replacing them with new low-flow formats. This conclusion came from the experience of regenerating old columns and learning how to take care of them.