Hello,
I am extracting polar compounds from organic samples using SPE. After loading the sample, I use pure methanol to elute the polars. However, I believe alkyl amines are being retained on the column due to the acid-base interaction. Is there a modified solvent I can use to elute the amines? I have read that mixtures including ammonia and/or CH3Cl can be used but I have not found any literature on this technique. I have further read that it is possible to neutralize the column before loading but again, I would like a paper to explain this method further. Thanks for any help!
I think you can use a solution containing methanol and an acid or base to adjust the pH of desorption solvent in the range of 4-8. This can help the process of desorption by changing the charge of sorbent and sorbate under appropriate conditions
I'm assuming you are using bare silica solid-phase extraction (SPE) cartridges (and not C18 bonded to silica). SPE is s simple type of chromatography.
A review of chromatography maybe helpful-
- A strong solvent is one that causes the compound to elute from the column (SPE in this case).
- A weak solvent modulates the strength of the strong solvent.
- On silica, less polar compounds elute first; more polar compounds need a stronger solvent.
- You can change the retention of a compound by changing the column or the solvent. For example, you can use alumina instead of silica. You can also use a stronger solvent (for silica, stronger solvents are more polar. That's why I stated my assumption earlier. Methanol and water are stronger solvents).
- You can also change the retention by adding modifiers to the solvent. These modifiers can affect the compound that you want to purify. For silica, they can sometimes affect the stationary phase.
Adding ammonia is intended to keep the amines as free-bases. It also tends to "deactivate" the silica, since silica generally has an acidic surface.By making the amines free bases, they are less polar and might be eluted with a weaker solvent. That is the reasoning for part of the reply above. Keep in mind that polar basic mobile phases dissolve silica gel. This starts as low as 10-20% methanol in dichloromethane. One can also use acid, which doesn't dissolve the silica. The amines will be ionized (and tend to retain), but stronger solvents can be used to elute them, including acidic water. Do a search for HILIC.
If you have an idea of what amines are stuck, try to get some standards and test those so that you can verify the method that you use causes them to come off the SPE cartridge, and also allows you to measure recovery.
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