High vs low exchange capacity SAX columns

[DJ]

Seems the lower exchange capacity column ought to offer the same N, ability to use weaker eluent for decreased background noise, and throw late-eluting anions, such as sulfate, phosphate back into the retention time window as other common ions, like Bromide and chloride.

So long as the weakest-retained anion in retained (say, F-),, why would one prefer a higher exchange capacity anion exchange column over a like column with, say, half the exchange capacity?

If there are instances were a higher exchange capacity column is more desirable, at what fold difference in capacity does this difference become non-trivial?

[tom jupille]

There are several parts to the answer.

First of all, for analytical HPLC A low capacity packing (say, up to about 50 microEquiv/mL) is about right. If you want to do prep work, though, that will have a limited loading capacity.

As a matter of history, pre- HPLC (and ignoring biomolecule applications) most ion exchangers were based on polystyrene, a material on which it's a lot easier to get full quaternization than it is to control at a lower level. As a case in point, it took Hamilton a good 5 years to develop a reproducible low capacity PSDVB anion exchanger in the early 80's to compete with the Vydac silica based material

[DJ]

Thanks, Tom.

By "preparative" work, where the higher exchange capacity column is desirable- could you elaborate such a real-life scenario, How commonly it would be would be employed (as opposed to, say, standard analysis of halogenated anions in municiple drinking water typically done with an analytical or smaller column were there is "hurting for signal."

Finally, at approx what delta increase fold difference in exchange capacity does this begin to make a difference?