Chromatography Forum

Is there a big difference between heptanesulfonic acid and octanesulfonic acid when it comes to ion pair reverse phase chromatography in HPLC?

I developed a usable method using heptanesulfonic acid but I ran out of the stuff. I ordered more, but I'm just curious about the difference. My guess is that the extra length makes it more hydrophobic and will increase my retention time.

I would guess the difference would be negligible.

It's discussed in good HPLC books, eg Practical HPLC Method Development, 2nd Ed, Snyder, Kirkland, Glajch, Section 7.4.2 on p326. It's important to work in a suitable [IP] range that matches desired column uptake of ion-pair agent.

A rough guide is ( assuming the same composition of mobile phase with suitable [IP] ), as sodium sulphonate carbon chain length increases by 1, decrease IP concentration by half to obtain similar separation.

There are lots of exceptions, but it helps me when I'm developing methods.

Cool thanks, Bruce. We have Snyder in our lab. I will be sure to read the chapter on IP chromatography.