Chromatography Forum

Firstly what is an eluent? Is it just another word for the mobile phase? I was reading about ion chromatography and noticed that for cation chromatography, the "eluents" used are often mineral acids such as HCl or HNO3. Why are acids used as the mobile phase? Is it to ensure that the cations remain ionised?

Yes, "eluant" = "mobile phase" (you say "poe-tay-toe", I say "poe-tah-toe")

The mechanism of ion exchange is just what the name implies: exchange of ions between the eluant and the stationary phase. So, if you are doing cation exchange chromatography, you need to exchange your (sample cations) for other cations. It turns out that hydronium (H3O+) is generally the most convenient cation to use in the mobile phase.

Thanks, that clears up a lot but I still don't have a complete picture. So lets say I'm doing cation chromatography and my stationary phase consists of sulphonate ions bonded to the silica support. I want to separate sodium ions from the solution. Is the purpose of the mineral acid to pair the sulphonate anions with hydronium cations so that when the sample is passed through the column, sodium ions replace the hydronium ions? Is that it?

"Sodium replacing hydronium" is the starting point. Actuall there is an equilibrium between sodium and hydronium cations bound to the sulfonate. The following eluent (after sample injection) will move the sodium though the column accordingly.