Chromatography Forum

I have tried pH 7 phosphate buffer on a C18 column labeled with pH range 2 to 7. And unfortunately, I had not seen that one of the vendor's slides particularly said: The column would deteriorate quickly at pH 7 phosphate buffer.

I selected this column because this column retains well 4-aminophenol under typical RP conditions without IP reagent but most others do not. My method works just fine except the column died (peak splitting) at 150 injections (method with 20 min run time, 30 oC column temp)

My question is if it is possible to extend the column life time to 500 injections if I low pH to 5.5 ? Did anyone do some study about the relationship between pH and column life?

It is known that at high pH the stationary phase will hydrolise. Most column manufacturers do pH test of the materials, ask them. Trifunctional bonding is more stable at low pH ranges, monofunctional bonding is more stable at high pH ranges. Please google for this subject at K.K. Unger, Uwe Neue, J. Kirkland, J. Doland and others. You will find tonnes of publications. And please bear in mind that pH is a function of temperature. When you adjust your pH at room temperature and your column is at 30°C the pH on the column is higher than before the column at room temperature.

Thanks. Gerhard.
For that case, when we design a method，should we consider the temperature effect as well as organic solvent? For example, I want pH 7 for pKa 6 analyte, use pH 6 buffer believing the temperature and solvent effects can raise pH to 7.

First off all you should be careful when selecting a column, I mean what column surface chemistry you will use. C18 is never the same. What pH you will use will be a result of some experiments. When your target compound has a pK from 6 you should look at pH8. If pH will be too high than look for a pH stable RP column where the column material is a polymer. High pH up to pH11 is no problem for such a column. If selectivity is ok than why not.
150 injections is ok. Higher number of injections would be nice, but please take into consideration how much time and money you need to invest. Change more often the column, that will save you time. In the meantime you can work with your method and earn money. Good luck.