I am trying to do citrate/citric acid assay by ion chromatography. I have a new ion exclusion column, using 0.003 N sulfuric acid and conductivity detector, and the citric acid elutes very close to the void volume. I am also assaying formate and would like to do both in one run; the formate peak is well resolved here and not an issue. Even when I make up standard citrate in the mobile phase, it doesn't help much.

Try increasing the sulfuric acid concentration (assuming that you have enough baseline offset in your conductivity detector). That should increase the retention time somewhat. If the conductivity detector runs out of steam, low-wavelength UV might be a viable alternative.

-- Tom Jupille / LC Resources