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Chromatography











Running a Column

Plan:

  1. Determine your solvent system. Usually finding a solvent system that moves your pdt an Rf ~0.3 is desired. If running a gradient you'll typically start your column with a system less polar and work up to the point of pdt elution.
  2. Determine size of column required. Typically you'll think in terms of the ratio of [silica weight] : [crude pdt weight]. Easy separations can be done with 30:1, harder separations 100:1.

Pack:

  1. Plug the hole above the stopcock with cotton.
  2. Add a thin layer of sand on top of the cotton (~1/2 inch)
  3. Slurry the silica in your desired starting solvent (or mixture of solvents). Pour the silica slurry into the column. Gently pat the column as the silica settles to help prevent air pockets from forming.
  4. Add sand on top of the silica (~1/2 inch)

Load:

Wet Loading:

  1. Dissolve your crude mixture in an appropriate amount of solvent. Less polar solvents are better (DCM is a common choice).
  2. Add the dissolved mixture to the top of you column making sure to distribute it evenly. Allow the solvent to completely descend into the silica gel layer.

Dry Loading:

  1. Dissolve your crude mixture in any solvent (needs to be a solvent you can remove by rotovap).
  2. Add silica to the dissolved compound solution. Typically it is recommended to add three times the weight of silica relative to your crude pdt weight (ex. for 1g crude add 3 g silica).
  3. Concentrate the mixture to dryness then add the resulting silica/crude pdt mixture to the top of the column.
  4. Add sand on top (~1/2 inch).

Elute:

  1. Start eluting with a solvent system which moves your pdt Rf ~0.1.
  2. Collect fractions of appropriate size. Check fractions by TLC as you go.
  3. Increase the gradients slowly until your compound elutes.



Common Solvent Systems


2 Solvents Systems:


MeOH/DCM (ex. 1:9)
EtOAc/Hexane (ex. 1:1)
EtOAc/DCM (ex. 1:2)
EtOAc/MeOH (ex. 19:1)
DCM/Hexane (ex. 1:1)

3 Solvents Systems:

DCM/Hexane/MeOH (ex. 7:7:1)
EtOAc/Hexane/MeOH (ex. 7:7:1)
EtOAc/DCM/Hexane (ex. 1:1:1)
DCM/Hexane/Acetone (ex. 2:2:1)

2 Solvents + Additive:

MeOH/DCM with NH4OH (1:9 with 1% NH4OH)