Exchange capacity and theoretical plates

[DJ]

I understand lower exchange capacity IEX columns have the advantage of improved detection by virtue of requiring weaker eluent for elution, hence improved signal to noise.

What are some other practical differences between high and lower exchange capacity columns?

I imagine higher-exchange capacity columns have more plates than like columns with less capacity, but what is the threshold for fold difference in exchange capacity, above which differences in theoretical plates begin to become measurable and significant? For instance, how much difference in performance and theoretical plates would you expect between 2 equ/unit and 1 equ/unit columns that are otherwise identical?

[tom jupille]

Efficiency (plate number) is not directly related to ion exchange capacity; it has more to do with particle size and particle size distribution. For a given particle size, it's also affected by the morphology of the packing (for the PS-DVB resins you work with, below about 12% crosslink are "microporous" or "nonporous"; above that they tend to be "macroporous"), the pore size distribution, and the surface area.

Pore size and surface area *can* affect capacity, so there is some correlation, but it's more "co-variability" than "causality".

In any case, when you're dealing with ion exchange, relationships tend to be "logarithmic", so a factor of 2 in capacity is hardly noticeable. To put this in perspective, a lightly sulfonated cation exchanger might be down around 10 microequivalents/mL, where a fully sulfonated material would be at 4 milliequivalents/mL -- a factor of 400x difference. I worked with both (both about 8% crosslink, and about 9-10 micron particle size) in the late 70s / early 80s, and my recollection they gave about the same plate numbers.