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Following up on How to perform solvent extraction of a solid I realized that I haven't quantified the size of the contaminants. I know they aren't soluble in acetone and the target is, but I don't know how small the contaminants are.

Once the target is dissolved presumably I must assume that suspended contaminants are as small as their molecular size. In this case is a filter-based extraction still possible? I don't see lab filters like extraction thimbles sold by filtration size.

In a case like this is the correct approach to run the solution through a centrifuge and then suck out the middle solution with a pipette? (Or what is the lab best practice for mechanical extraction following spinning?)

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There are two approaches regarding the size selectivity of solid to be filtered. Most common is some sort of "tortuous path", that is a fine weaving of some type of fiber having some average size cutoff regarding what gets filtered out and what passes through. Secondly there are filters that are actually grids of a specific size of hole or square to restrict the solids from passing through. These look like a screen sieve through a microscope and I've seen them as small as 1 micron, though this is extremely small and usually used for sterilization.

In reality, you are more likely to have problems with your filter becoming clogged than with too much solids getting through. Your idea of centrifugation is good, but as a pre-filter step to help prevent clogging the filter. In this case you could either decant or suction off the bulk of the liquid before the finer filter finish. The requirement for final filtration size is application dependent.

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