I am having great difficulty separating the following compounds: acetylenedicarboxylic acid, ketomalonic acid, fumaric acid, tartronic acid, tartaric acid, maleic acid, oxalic acid, dihydroxytartaric acid(salt), and dihydroxymaleic acid. I have tried an "organic acids" type column using 0.005N sulfuric acid mobile phase, but many are not separating quite well enough to quantitate low levels (if all are present). I have also tried a C18 column with 20mM Pot.PO4 buffer, ph3, but some compounds do not elute. Does anyone have experience or information concerning this challenge??

I can't say for sure because I haven't tested all of the analytes you listed but the majority can certainly be separated best by utilizing anion exchange chromatography. Generally speaking, ion exclusion columns don't exhibit very good divalent carboxylic acid selectivity, so I'm not surprised that you couldn't perform the separation successfully with that mode. On the other hand, I'm a bit surprised you can't elute all of the organic acids from a C-18 column. Generally, reversed phase separations in the ion suppression mode don't perform very well for divalent carboxylic acids but usually the problem is resolution not elution. Anyway, the selectivity of anion exchange increases as a valency of the ion increases, so I think anion exchange is your best bet (ion pair chromatography is also weak as far as divalent ion selectivity is concerned). A good first choice would be the AS11 column from Dionex with a hydroxide eluent using a suppressor to remove the hydroxide postseparation. Conductivity will provide the best sensitivity but UV can also be used after the suppressor.

If you need more definitive information please let me know. Although, I'm pretty sure we don't have all of the organic acids you listed in-house so it may take a while to verify which column works best (unless you want to send me your standards).