Chromatography Forum

Hi ,

Quick query

Any significant difference between a C18 and RP18 column (symmetry)?

Cheers

Yes. And no. It depends .
There MIGHT be differences depending on the manufacturer of the column. Unfortunately, there's no clear nomenclature. If you have a "C18" column, you can be pretty sure that it actually is a plain vanilla C18 column. For a "RP18" column, thinghs might be trickier.

Are you referring to a Waters Symmetry column? That's a good example of the trickier case: "Symmetry C18" is a real C18 column, "Symmetry Shield RP18" (note the "shield"!) is a polar embedded "C18-like" column.
On the other hand, if you look at a Merck "Lichrospher RP18", that's a real C18 column, despite the "RP18".

Bottomline, have a thorough look at the column brochure, or ask the manufacturer.

Yeah ,

Symmetry C18 vs Symmetry shield RP18 .

The selectivity and reproducibility would be the same ?
Wouldn't RP18 improve peak shape?
Don't normal C18 experience phase collapse under high aqueous conditions?
I am just trying to justify the use of RP18 instead of C18 ,,is this easily done?

- Yes, a polar-embedded C18 like the symmetry shield is resistant against phase collapse/dewetting in contrast to the symmetry C18
- Selectivity CAN be quite different depending on your application
- Reproducibility should be the same
- Peak shapes CAN be better on the shield RP18 because of suppresion of silanol interactions, especially for basic compounds. No guarantee, though.

Summarized, Symmetry C18 and Symmetry Shield RP18 are different columns!

That said, I've already switched in the past between a "C18" and a polar-embedded "RP18", denoting both as "L1" and thus equivalent. It's not reaaaaaaaally legitimate, but I had no troubles with our regulatory guys, authorities or auditors so far. The missing nomenclature helps here . The new column should ace system suitability, of course.

C18 is never the same, same with RP18. Do you call a car car or automobile?
Counting the C atoms it is a count of all the C atoms, also in an embedded group. That means that the alkyl chain is not long and don't have 18 C atoms in it.
Most reliable and respected manufacturers will show you what silane they used to bond on the silica surface so you can see it and you will know how many C atoms you have. But more important is the hydrophobicity table to see what differences are with different brands. Also there is no standard procedure how to measure the number of C atoms. Carbon content is only for a very few number of applications important, for example C30 for carotenes.
Why C18/RP18????????? It was just by accident. Silane was available on the market, high purity and not too expensive.
Now we have C16 or C20 or what so ever on the market.
What ever you use - it should give you a good and reproducible separation!
And note: there are no bad columns on the market, it all depends on the application.