- A convoluted extraction series.
- Change the limiting reagent.
- Distillation.
- Trituration / Sohxlet.
- Prolonged high vacuum treatment.
- Recrystallization.
- The bisulfite/DNPH trick (aldehydes only).
- Secondary reactions [1].
- HPLC.
- The next reaction in the series.
[1] Protections, oxidations, etc. Ex. Contaminant an amine? Treat with acetyl chloride and base. RF shoots up, and the column is no longer difficult. Transient protections are also useful for 1,3-diketones and other tautomeric structures.