Pore size question

[Mike H.]

I'm working on a method that has a pair of co-eluting peaks. This isn't the problem since I learned enough from one of Tom's classes to correct this. What is very curious is that the co-eluting pair has very different structure and sizes (MW500 and MW800). I don't understand how co-elution could be possible with such drastically different structures and sizes. BTW I'm using a phenomenex EVO 100mm column. 100 A.

[Gerhard Kratz]

In RPC you have other parameters which are responsible for a separation/resolution. MW difference is not so high and you are not doing SEC.
Maybe a HILIC column will get a better separation if both compounds have different charges.

[DR]

The physical size and shapes of analytes play only into the tortuosity and diffusion (physical) portion of the resolution equation. The physical portion is typically the least influential of the 3 portions of the equation that govern separation (resolution). Changing alpha (a stationary phase or significant mobile phase change) will be most likely to separate your co-eluters.

Remember, (most forms of) chromatography exploit differences in partition coefficients between analytes, not size and shape differences. Even though they exist, size and shape differences may not result in any differences in dipole moment as far as a given mobile/stationary phase system is concerned.

[Mike H.]

Thank you. The insight is very helpful.