You may find this question silly, but I suddenly fell in doubt as to whether a "C18 column", an "octadecylsilyl column" and an "octadecyldimethylsilyl column" are synonymous.

Or are they synonymous only if the column material has been endcapped during production? And aren't they all nowadays?

Looking forward to an "of course..."-answer!

Kind regards

Eva Horn Moeller
Assistant Prof.
The Royal Danish School of Pharmacy
Copenhagen
Denmark

I assume that you ask in the context of a description of a column for the analysis of a pharmaceutical. In this context, all these descriptions are synonymous. However, this does not mean that an analysis performed with columns of this category will be identical, but the description is not the issue, rather the broad range of packings that classify under this category.

Thank you for the kind answer. Well, the precise situation is that I wanted to compare a column from the literature (octadecyldimethylsilyl, probably packed by the researchers themselves) with a Supelcosil LC-318 column which we have in the lab. Knowledge of the pore size and particle size of the columns was accessible, but whether the Supelco material was octadecyl- or octadecyldimethylsilylated (endcapped, as I understand it?) silica, I could not establish. No mention of it in the column data sheet or at the company homepage. And then I just got plain curious. Now I have contacted Supelco - let us see if they know.

- The two columns are a little different in the silica quality, so it will be trial and error anyway. The job: I need to analyse and separate BOC'ed insulins. Should be interesting.

Kind regards
Eva

Hi Eva,

I think the octadecyldimethylsilyl refers to the bonding chemistry used to form the siloxane bond not whether the column is end capped or not. The "stable bond" columns use diisopropyl or diisobutyl instead of dimethyl.

TMS is typically used as an endcapping reagent to reduce the silanol activity and give better peak shape for basic compounds. However, this endcapping reagent hydrolizes easily so is not used in low pH applications.

So I think "regular" C18 columns are octadecyldimethylsilyl and can come either endcapped or not.

Hi Eva,

Just to amplify what Tom said, an octadecylsilyl column contains the group

-O-Si(R1)(R2)C18H37

where R1 and R2 are not particularly specified.

The term "C18 column" is a less informative jargon for the same thing which refers to the C18H37 group. A dimethyloctadecylsilyl column has R1 and R2 as methyl groups, although as Tom says above, there are other possibilities. Endcapping is a separate matter and refers to further reaction of previously underivatised silanol groups with a less sterically hindered reagent.
There are often many differences between columns that are superficially similar. Thes may occur for example from the purity of the silica used to make the column or the amount of reagent bonded to the column. Thus you may not get the same separation, even on two endcapped dimethyloctadecylsilane columns obtained from different manufacturers. This may be slightly problematic for you if you cannot obtain
the same column as used by the original researchers. However, I doubt if they made the packing themselves (very unusual these days!) and if they packed the column themselves from commercially available sources, then you could probably buy a ready packed equivalent.

Mange hilsner,

David

Thanks all, I am grateful for your kind help.

Supelco answered that "The Supelcosil LC-318 is octadecyl silane and it is
endcapped", so I guess I'll take that as stated...

Kind regards
Eva

Eva,
Bonded phases have resulted from what starting materials were readily available, rather than some theoretical considerations. Coke Corp. coated their bottles with ODS and so the material was readily available. ODS comes mono, di, and tri reactive, i.e., the reactive group replaces the silanol and the reactive silane can have up to three leaving groups. Using multireactive silanes gives a polymeric coating, thick but had to manufacture and with few advanges.
The trisubstituted forms a mono layer on the silica. Large side chains sterically hinder the reaction so more silanol groups are left exposed. No good comes and so almost all manufactures use dimethyloctyldecyl.
Jim