Difference between heptanesulfonic and octanesulfonic acid

[MestizoJoe]

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.

[Consumer Products Guy]

I would guess the difference would be negligible.

[Bruce Hamilton]

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.

[MestizoJoe]

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