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TLC Optimization
Prior to running a sample on a flash instrument/column it is important to determine the optimum solubility, selectivity and resolution by TLC. The following solvent systems provide a wide range of selectivities and are a useful screening tool. Valuable information is obtained by running the sample in several mobile phases and will improve the odds of obtaining purity using Flash. It is also useful to create a weak and strong polarity for each of the mobile phases to obtain additional data points.
The TLC plate which best correlates to puriFlash HC & HP silica columns is part PFT-1000. For additional information, click here.
| Solvent System | Compound Characteristics | TLC Conditions | Comments |
|---|---|---|---|
| EtOAc:Hexanes | Lipophilic molecules without ionizable groups | Start at 1:1 and adjust until Rf of target is 0.2 - 0.3 | Mixture applied to flash column should be soluble or partially soluble in the mobile phase. Adding DCM will aid solubility but reduce retention. |
| DCM:MeOH | Lipophilic molecules without ionizable groups | If above conditions do not move target at 100% EtOAc, add MeOH to DCM at 1% up to 15%. | If you have an ester functional
group in your product, transesterification
is possible. This happens
when the product ester is xchanged
with the solvent alcohol
used in the mobile phase - if you
have an ethyl ester in your product
and you use MeOH in the mobile
phase you may end up with a small amount of methyl ester in your product. Silica can be acidic and will promote this type of reaction. |
| DCM:MeOH + NH3 | Molecules with basic amine functionality | 90:10 DCM:MeOH and adjust MeOH until Rf of target is 0.2 - 0.3 | It is common for streaking to occur with basic amines. To reduce streaking/band broadening add an amine modifier (a few drops of NH3 or a solution of NH3 in MeOH). Adding an amine modifier can improve compound recovery from the column. |
| DCM:Hexanes: MeOH + NH3 | Molecules with basic amine functionality | 90:10 DCM:MeOH and adjust Hexanes until Rf of target is 0.2 - 0.3 | If you need to use DCM:MeOH but it is still too polar you can add hexanes (or replace the DCM with hexanes). This will improve the Rf without changing the nature of the column and retain the solubilizing power of the mobile phase. |
| Diethyl ether: Hexanes or THF | Non-polar molecules | Start at 1:1 and adjust until Rf of target is 0.2 - 0.3 | Use methyl tetrahydrofuran instead of THF which does not readily form peroxides. |