Chromatography Forum

Hello,

does anybody know what might help to improve the peak shape of a peak that looks like "chair"? I tried to increase the pH from 5 to 5.5. Do you believe a higher pH will help?

Thank you in advance

Maybe? maybe decreasing will also help, unfortunately we have no idea without much more info, if you could post some info such as molecule under analysis and method conditions we could help more.

cheers
Chromavore

Unfortunately, I cannot give more information. The molecule is acid and the mobile phase is buffer/Methanol/H2O: 10/30/60, v/v/v

Then we cannot really help you.

I have often seen "chair" shaped peaks at the beginning of the chromatogram (early eluting peaks) when the injection solvent was stronger than the mobile phase composition. Maybe this applies to your case.

Hello Snis,

I work with a variety of acids and I think I can help you here. In this case, your analytes are acids but your mobile phase is not acidic enough to fully protonate your analytes. While we are lacking a lot of information from the post, you should add a stronger acid modifier to your buffer (something like 0.15% formic/acetic acid, if already at 0.15% then maybe take it to 0.2%). Typically, when you have ionized acids in your chromatogram, they will elute early, with peak-splitting that is due to the proportion of ionized and non-ionized analytes traveling through your system. If this is a prep separation (I do many of these) you may see a chair because your injection solvent may not be strong enough to suppress ionization or you could be injecting too much (conc or vol). I would suggest above all else to get the pH down and see how that affects your results.

TS

Hi Snis

For sure the solution chemistry is critical to observe a good peak shape.
However last week I had the same issue caused by the tubing between the column and detector.
An operator in the lab had replaced the original item with a wider bore tubing that caused broad and chair shaped peaks.
Installing the the correct outlet tubing restored the chromatography.