Chromatography Forum

Hello,

I'm working on a substance that has two phenyl rings. I thought I could use a phenyl column (acquity CSH phenyl-hexyl, phenomenex F5) to maximize my selectivity, but all i got was unacceptable tailing.

Its a water soluble substance with a negative charge and two phenyl rings
Buffer was 15 mM and 50 mM PO4 in water @ pH 6.6 (away from any pka's)
Organic was MeOH and ACN was also tried and both gave tailing > 3

Can anyone advise on what went wrong?

Thanks,

I only tried phenyl column once, didn't work for me. Try RP-18.

In order to benefit from pi-pi interaction you need to use MeOH, use of ACN diminishes this interaction. If you compounds are basic, the tailing might be attributed to overloading of residual silanols. You can try to use lower pH (pH 2) in order to suppress ionization of these silanols or inject less. The problem of overloading does not exist in mixed-mode chromatography. Please let me know if you have ionizable groups in your aromatic compounds. May be instead of exploring pi-pi/RP mechanisms you can explore HILIC/IE or RP/IE.
Here are few examples on separation of aromatic structural isomers:

https://helixchrom.com/compounds/m-toluidine/ (toluidines)
https://helixchrom.com/compounds/2-aminobenzoic-acid/ (aminobenzoic acids)
https://helixchrom.com/compounds/4-aminosalicylic-acid/ (aminosalicylic acids)

Thanks guys. I did switch to a mixed mode c18 and it is working.

I verified that the tailing is due to pi interactions. Feeling kinda bummed. Maybe phenyl columns are for molecules that have non-phenyl pi orbitals.