Chromatography Forum

Dear all,

in recent years, HPLC column selectivity has been increased and many modified C18 columns are available.

Is there a rationale, in which cases an embedded polar group shall be used and in which cases a column with polar endcapping might be successful ?

Thank you very much for your advice.

Regards

Florian

Contact me and I will send you three publications on the subject of selectivity.

Uwe.Neue@Prodigy.net
Uwe_Neue@Waters.com

Some companies have a different view of HPLC. I suggest having a look at this article:

A Novel Multi-Mode ODS Column Which Consists of Anion Exchange,
Cation Exchange, Normal Phase, and Reversed Phase Mode
I.YAZAWA. B.Evans. LC GC North America. The Application Notebook. June 2009

Up 'till now, C18 phases (including both polar endcapped and polar embedded groups)
have struggled to retain / separate polar molecules.

This multi-mode ODS column expands the separation possibilities of the C18 phase.

Brian,

I like this "up until nowâ€

The question was about embedded polar or polar endcapped phases, and not about other things...

Generally, embedded poar phases or "polar endcapped" phases can be used in very polar mobile phases, including 100% water. This is something that can't be done reliably with many C18 phases. You also should realize that many "polar endcapped" phases are simply not endcapped at all, and contain plenty of silanols as "polar endcapping".

There are significant differences in selectivity compared to C18 phases. These come from the embedded polar group or from the endcapping. For example, EPG phases with amide or carbamate groups give stronger retention for several types of analytes that can hydrogen-bond to the EPG group, such as phenols, non-ionized carboxylic acids etc. Also, the polar group can suppress interaction of basic analytes with surface silanols. Thus many phases with an embedded polar group give very good peak shapes for basic analytes.

Some of these EPG phases are made in a 2-step bonding process that leaves amine functions on the surface. Such packings have problems with the stability of retention and should be avoided.

The original post was about HPLC column selectivity, C18, polar embedded group, and
polar endcapping. The article I referenced compares a novel C18 vs. a "polar endcapped"
(hydrophillic polymeric endcapping) C18 phase. I believe it is related to the discussion.

Vlad - Imtakt has 10 yrs. experience making 3um silica. They know what they're doing.

I am not questioning ability of Imtakt to make 3 um silica, just question of integrity of the statements in your articles...."up until now" and another one that I liked ""The RP/anion/cation phase columns that are marketed today consist of only one ligand structure" ...even some companies have multiple phases for years including, but not limited to Alltech/Vydac, Dionex, SIELC. Alltech had couple of mixed-mode columns for over 15 years

We, at SIELC have a t least 6 different ligands. DIONEX and Alltech have at least 2 each.
Also you can have groups on the same silica particle if you separate them by long enough chain.
When you mix 2 or 3 silicas no matter what you do the rate of hydrolysis/aging/deactivation will take care of long in term reproducibility in a bad way. When you have everything on one ligand and you hydrolyze it relative ratios between properties does not change to the same extend when you have a physical mixture. So you peak might shift but most likely to the same extend.

Please, please: when I see a flame-war going on between two or more column manufacturers, it makes me want to buy my columns from someone else altogether. This isn't a good sales tactic, guys!

But it can be quite instructive!

Please, please: when I see a flame-war going on between two or more column manufacturers, it makes me want to buy my columns from someone else altogether. This isn't a good sales tactic, guys!

I understand. And I will do my best to not turn this into a discussion about our products.
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To reiterate, the article I referenced compares two (USP L1) C18 phases.
And it's important to understand columns branded as "xyz - RP" are not
C18 phases. While I understand "non-C18 RP ligand + IEX" columns can be useful,
I don't understand how "non-C18 RP ligand + IEX" columns are related to a discussion
concerning C18 phases.

Perhaps it is best I reserve commenting on "integrity, novel, and durability"
for future discussions (since such a discussion could be viewed as unrelated to
the initial post).