I'm working with a compound with pKa 2.3 and 9 (a third one is -3.5). Could someone kindly suggest a HPLC column, mobile phase and elution method?
Thanks!

To make a usefull suggestion it would be very helpfull to know a little bit more about the compound.

Is this an aliphatic compound? Is it a small and very polar compound or is it very big with only 3 polar groups ? What kind of system do you want to use: LC or LC/MS? Do you want to use a RP column or an ionexchange column ? What have you allready tried ?

Best regards

Thanks for reminding me in time!
The compound can be regarded as an aromatic with three N's (one of 1" and two of 3") on the rings and a -CN group. It's not a big molecule (MW 200). I have tried SUPELCO RP C16 and Cyano on a HPLC with 5/95=ACN/pH7 PO4 buffer. The Cyano gives a RT around 6 min not that bad, but we wish to increase the RT given the studies we are doing) while RP C16 elutes it at about RT 3 min (to earlier for the study case!).
Again, I appreciate any suggestions in advance!

Reduce the ACN to nothing

If the group corresponding to the pKa 9 is a basic group, then you might work at higher pH. At pH 7 it is protonated and less retained. At pH 9 or higher, it is less ionized and will have more retention.

Waters Xterra and Agilent Zorbax Extend columns may be used up to pH 12.

Good luck!

If the molecule has NH or NH2 groups, you might want to try using a ZirChrom-PBD column with a basic mobile phase.

I would try using a Zorbax C8 or C18 column to start(5um, 4.6id, 7.5cm or 15cm long for potency or 25mM for rel subs).
pH 6.5 25mM phosphate buffer (may add 0.1% TEA for peak shape if necessary).
gradient method starting at 5 to 10% acetonitrile to 90% (maybe a 5mins hold at the start). Perform a shallow and steep gradient followed by drylab analysis to optimize your condition.