Changes in k with C18

[flozbi]

Hello all.

I'm developping a method to separate a mixture of isomers. It works well with C18 column.

I must scale-up my method.

I have observed that by using always C18 columns, but by increasing the particle size 5 to 10, 10 to 20 micro my rentention factors increase ( I use the same isocratic eulent).

My question is why ?

I thought that the retention factors will stay the same, but that my separation will be less good (cruder) !

Why am I wrong ? Can you explain ?

Thank you

[tom jupille]

If you have identical stationary phase chemistry, you should get the same k' regardless of particle size . . . but that's a big "if".

The most obvious question to ask is "are all three columns the 'same' packing (i.e., same brand and product name)?". If not, then there is no reason to expect the same k'. When we describe a stationary phase as "c18", we are specifying only one of at least a half-dozen parameters that affect retention (pore size, surface coverage, silanol acidity, side-chain, mono- vs. di- vs. tri-functional bonding reagent, . . . I've probably missed some, but you get the idea.).

Even if the packings are nominally the "same", it's not unusual to see lot-to-lot variations in retention. I'd be surprised if you got three different particle-size columns from the same lot of packing material.