Supposedly you can extract sucralose from splenda with pure grain alcohol. The non-sucralose stuff will settle to the bottom and you can pour the top off. I havn't tried it or heard of anyones results from trying this.
You're better off buying pure sucralose and mixing your own IMHO.
Bumping old post.
It's called fractional crystalization, you dilute your stuff (whatever it is) in a suitable solvent (water, alcohol, petrol, depleted uranium etc etc)
Boil the solution till all dissolves, then let it cool, crystals or powder will form on the bottom of the beaker. Discard the solvent and re-dissolve the crystals/powder and perform the same steps 3-4 times.
After that let the powder/crystals dry and you will have a purified product.
Chemists do this all the time and yields a pretty clean product.