Chromatography Forum

All else equal, under what circumstances would one choose a WAX over SAX for protein/peptide separation? If the pH is held constant (below pKa of WAX) and an increasing salt gradient used, are these two columns the same?

Do you mean LC or just cartridge purification?
Don’t forget that this is not an “ideal gas”. I mean it is not just charge attached to nothing. There is a support Si or polymer that also contribute to separation of analyte and rubbish. And there is always a manufacturer/ batch quality/consistency/impurities behind the product. With WAX you have more options as quaternary amines are always charged.

HPLC

I understand WAX could become uncharged when the pH is increased... I just don't see a lot of folks running linear pH gradients these days. The SAX column, on the other hand, has broader pH range. You seemingly could drop the pH and elute peptides, proteins from SAX as easily as WAX. All other things equal (assume polymeric support chemistry is same), I don't see why one would prefer WAX over SAX.

SCX vs WCX I get...- But WAX vs SAX confuses me

DJ -

You are correct that a WAX material loses charge density in a continuum from pH 5 to pH 9.5. SAX materials retain their charge density over that range. Now, one reason to prefer a WAX material is that the additional alkyl- groups that are attached to a SAX ligand increase its hydrophobic character. That's significant in many biological applications; recovery of delicate proteins is sometimes affected by hydrophobic character of a stationary phase. Above pH 8 a WAX material has decidedly less charge than a SAX material. That said, you wouldn't use an ordinary silica-based material above pH 8 anyway, so it's a moot point unless you're using an anion-exchange material based on a polymeric material or an inorganic material such as titania or zirconia. Those should be avoided if you can help it; peaks tend to be a lot wider on such materials than with a good silica-based material.

Do you have a specific application in mind that would be conducted at a pH > 8?