separation of ammonium from diammonium compounds

[schnippy]

Dear Chromatographers,
I try to find a suitable procedure to separate ammonium from diammonium compounds in a preparative scale. Is it possible to use an Amberlite or Dowex ion-exchange resin in combination with an pH-or NaCl-gradient?
Thanks in advance for yuor help!
Schnippy

[Chris Pohl]

schnippy,

Such a separation should be fairly straightforward using either of the resins you mention. Generally, divalent cations exhibit significantly greater retention than monovalent cations so achieving a separation is simply a matter of adjusting the ionic strength to elute first the monovalent species and then simply step to a higher ionic strength in order to elute the divalent species. Either pH or ionic strength gradients can be used to achieve the separation but an ionic strength change will allow you to achieve faster re-equilibration assuming your electrolyte contains only one type of cation. If you employ a buffer, use an anionic buffer to avoid long equilibration times. Gel type materials of this sort are a bit slow in terms of mass transport so to use fairly low linear velocity. If your cationic species are hydrophobic, it might be beneficial to add solvent to your mobile phase.