Baseline separation of benzaldehyde and acetophenone has not been achieved using typical chromatographic conditions. I have tried several types of C18 columns. The use of acetonitrile vs. methanol gives similar results. Is there a stationary phase well suited for these compounds?

Have you tried the reversed phase columns especially designed for retaining polar compounds?
eg Waters Atlantis dC18, Alltech Prevail C18, ...

I have tried the Waters Atlantis dC18, Waters XTerra RP18, and Hypersil Hypurity Advance as polar retentive candidates. I am able to achieve retention, just not resolution.

I guess it can be done with a reversed phase stationary phase like RP18, C18.

Have you tried to increase the temperature?
Just make some injections with 2 different concentrations of organic phase, and 2 different temperatures (4 runs total). Now you are sure you can see the effect of these 2 parameters. If this doesn't help, try the effect of acid, but my guess is that this isn't neccesary.

You can make your mobile phase acidic with eg 0.1% phosphoric acid, TFA, ....
(see (p 12))


Good luck and stay in touch...

I have found that phenyl stationary phases have worked in cases where all manner of C18's and C8's have given inadequate selectivity (regardless of N or k'). Rumor is they work better with methanol as organic modifier, and that's what I've found anecdotally.

I have also experimented a little with a pentafluorophenyl phase, and found it gave different selectivity than either C18/C8 or phenyl. The one I used was from MicroSolv (www.microsolvtech.com) and called "F5". Thermo-Hypersil-Keystone also makes a couple of fluorinated phases, and I think one of them is the PFP.

I am highly surprised that these two compounds do not separate. What were the retention factors that you worked with?

OK, I'm no rocket scientist (well, maybe close) so I spent 45 minutes today on this, as a favor. All this "guessing" doesn't add too much. No, I didn't weigh anything out. On a traditional type "A" RP-18 column, I could readily separate these to baseline using 16% ACN, 8% THF, remainder aqueous with a little acetic acid in it (don't know if the acetic acid is necessary); benzaldehyde eluted first, to baseline, in a 4 minute run. THF can really help with stuff that co-elutes, wouldn't use it otherwise. I also used a specialty column and resolved them with 10% ACN, 10% THF, remainder acetic acid-water, but this was a 9 minute run, again benzaldehyde eluting first. I didn't try to post chromatograms, didn't think that work was necessary.

I obtained baseline resolution using 40% Methanol on an XTerra MS8 3.5µm column. This column contains ether linkages that may contribute to the selectivity. I was unable to use THF for my application due to other compounds with limited UV response. Thankyou for your suggestions.

Glad you got a separation. Just one correction: XTerra MS C8 does not contain any ehter linkages. It is a plain old C8 bonded to a hybrid particle. This hybrid particle contains methyl groups in the matrix, attached to the silicone atoms. This makes it more stable than silica at alkaline pH.

Uwe Neue,
I described the bonding of the XTerra MS8 incorrectly as "containing ether linkages", I was referring to the trifunctional ligand chemistry that produces three Si-O-Si bonds at the base of the ligand.