Peak shape problem

[chfoo]

I have a peak shape problem with phenylephrine using zorbax RX C8 columns on both 150x4.6 and 250x4.6. Lower injection volume was better but not great. The peak shape was good on other columns (Luna, Apollo, sunfire, kinetex) using the same solution and mobile phases (2gPIC 8 pH3, adjusted with H3PO4). pH2 and pH 2.8 and lower concentration of PIC8 and PIC 7 had also been used.

All other analytes have good symmetry factors.

Please advise.

[tom jupille]

Column chemistry is arguably the single most important factor in peak tailing problems. You have already tried the "quick and easy" fixes. If this were my problem, I would simply use one of the other columns!

[chfoo]

There are 10 analytes that I am trying to separate and this column separates them all with only 20 minutes run time. Can anything be done on the sample solution to improve the peak shape of Phenylephrine? I am still working on the separation of all these analytes in the other columns but so far critical pairs resolutions are not good.

[tom jupille]

To oversimplify, much tailing of bases comes from interactions with acidic silanols. The Zorbax Rx came on the market in the mid-80s. It was a very good column in its day, but there is a lot of newer and better (from the point of tailing!) technology available today. The catch is that those active silanols also affect selectivity.

You have already gone part way (lower pH). Other things to try are increasing the buffer concentration (up to 25-50 mM or so phosphate) or, as a last resort, adding a tailing suppresion reagent like triethylamine to your mobile phase. All of those things will also affect selectivity.

If you want to use one of the other columns, look at what your critical pairs are, exactly in order to manipulate selectivity. If the problem is with two basic compounds, they try tweaking the pH or the temperature. If the problem is with two neutral compounds, then try switching to a different organic solvent. If the problem is with a neutral and a basic compound, try tweaking the ion-pair reagent concentration.