What would be the proper way of cleaning solvent filters (from mold, particles, etc). I'm thinking about an aqueous acetic acid solution combined with sonication, then followed by a wash in methanol or acetonitrile. Any other suggestions will be considered.

The best thing you can do, is to replace it with new solovent filter.

Many years ago we used to boil the stainless steel filters in 25- 50% nitric acid (take proper precautions !!!!) then rinse thoroughly in deionized water. This essentially oxidizes away the majority of organic contamination (in our case mainly from bug growth -- we were using aqueous buffers). Having said that, I would concur with the preceding post: it's probably more cost-effective to just replace them.

Cost depends on the filter! HP charges $80 (if I remember right) for 1050 filters.

We plug the water filter once a year with bacteria or whatever crud grows in our water. I backwash it with a hose on the faucet to knock the loose stuff off, sonicate with MeOH, and rinse and backflush again. They work as good as new for many months. Take very little time.

We add 1-2% MeOH to our water to retard growth, and it helps a lot. I wash and SCRUB the water reservoir every month or so. Today I did the same to the MeOH reservoir because it had 'floaties' in it.

I find that sonicating in 35% nitric acid for 30 min followed by flushing with water repeatedly until pH of aqueous is over 4 works well. Finally sonicate in mobile phase to remove air bubbles etc and reconnect to HPLC solvent line.

HTH

Tony

I'm not sure whether porous stainless steel - the material for the most solvent filter - can withstand sonication, or boiling in nitric acid in a concentration over 6% (~1 mol). The price for a new, good SS mobile phase filter lies in the $ 10-20 range. (Upchurch, Jour Research…). It is better to replace it.
So called "Biocompatible" filters are made of inert polymeric components. Maybe they are more resistant against acids. (But I afraid they are sensible against oxidation…)

We have tried about everything mentioned above. except boiling nitric acid, on just about any type of stainless frit (including column frits). The low resistance of a new frit could only be restored by placing the frit in concentrated H2SO4 (room temp.) until color formation ceased. Plugging was due mostly to proteins and some other org compunds with hetero-atoms. Such material usually dissolves in conc. H2SO4.

One should mention that if the frit is encased in Teflon the con. H2SO4 method is still ok, but not if PEEK is attached. H2SO4 attacks PEEK.

The best method that I found to work, is scrubbing
your filters under hot water first. Then sonicate
them in a hot soap bath using Liquinox soap. Take
them out and rinse them under hot water again to
remove any residue. Then rinse them with
deionized water. Then sonicate them in a beaker
filled with acetone to dry them out and to
decontaminate them. Change out that solvent, and
replace it with methyline chloride and sonicate in
this solvent. Your filters will practically be
brand new and you won't have to worry about buying
any for a while.

I was told by my Waters service rep to boil the filter in 6N nitric acid, then rinse many times with deionized water. Sonication helps here. However, since I began filtering EVERYTHING that goes into the mobile phase, I haven't had a problem with my filters. This has been over two years, at least.