Hi everybody.
I'm a newcomer to the field of anion exchange. I just finished using a Waters Spherisorb SAX column with a mobile phase of ammonium dihydrogen phosphate, as required from the literature method I used. Since I do not plan to use the column for several weeks, I would like to receive some suggestions from you on the storage conditions, namely:
what solvent should be left inside the column?
should I use any special washing protocol in moving from 5 mM NH4H2PO4 to the long-term storage solvent?
Thanks in advance

I would remove the buffer portion of my mobile phase and replace it with water. For example if you have 10% 5mM NH4H2PO4/90% MeOH, then flush with 10% water/90% MeOH. You could theoretically store your column with this mobile phase.

I'm not sure it's such a good idea to apply standard reversed phase column storage practices to ion exchange phases. It's important to remember that the pH of the environment around the ion exchange sites are defined by the ions associated with those sites. Rinsing a phase previously in equilibrium with a buffer will shift the pH of the stationary phase in accordance with the nature of the buffer ion. For example, the buffer specified in the original question is relatively nonaggressive with regard to corrosion of the silica (although phosphate is generally one of the most corrosive, this property is quite pH dependent). However, rinsing with a "neutral" nonionic solution will shift the pH of the bound phosphate anions upward toward a pH which is much more harmful to the silica. I would say that a safer practice would be rinsing the column with a mildly acidic solution prior to storage. For example replacing the buffer solution with 10 mM acetic acid (keeping the solvent concentration the same as is used in the method) would prevent this stationary phase pH of problem without exposing the stationary phase to dangerous extremes of pH. Furthermore, since acetate anion is very weakly retained, it should be quickly displaced by the buffer upon resumption of use

Chris and Anon:

I didn't want to give false information, so I called the waters tech support hotline and they suggested washing the buffer off with water and then storing in a 100% acetonitrile for long term storage.

For long-term storage, the last suggestion is the one that is generally recommended. All silica-based bonded phases exhibit hydrolysis with time. Hydrolysis happens in water. No water, no hydrolysis. This is the reason for the long-term storage of silica-based stationary phases in acetonitrile.