High pH applications on silica-based columns

[DJ]

I'm trying to gain an understanding for what one can expect from an ODS column that is made for high pH applications (Phenomenex Gemini, Waters X-bridge)---


Efficiency:
Compared to original QA plate count, what efficiency can be expected under high pH conditions--- with well behaved and not-so-well-behaved solutes?

Seems column lifetime has to be shortened when operated at high pH, even for columns with improved stationary phase chemistry. Is this the case? If so, how rapidly does performance deteriorate?

[Gerhard Kratz]

At high pH hydrolysis of the SiOH groups will lead to short lifetime of your column. The better the surface bonding is done, or Hybrid materials are used, the longer the lifetime. But hydrolysis will happen.
Advantage is a better peak shape for Amines for example. Monofunctional bonding is better for basic conditions, trifunctional bonding is better for acidic conditions.
How efficiency is affected depends on the compounds and the buffer salts used in the mobile phase. Volatile buffers are highly recommended.
When you have installed a new column try to increase pH slowly. Out of my experiance that will help lifetime. Don't use this column for other pH values next time.
In some older literature you will find that a so called saturator column is used. You can install a plain Silica column, for example 4,6mmID x 3cm Length between Pump and injector. Hydrolysis will happen and mobile phase is "saturated" with desolved silica. But that has other side effects and cannot be used with ultra high pressure systems.
In most cases a guard column will have the same effect and it means that the guard column should be replaced more often than usually.
For extrem high pH values pH stable packing materials are available, for example C18 modified PSDVB. Such columns can be operated at pH11 for a long time. For sure efficiency is lower than a silica based C18 column. Peak shape is not so nice, but if resolution is ok you can work with such a column.

[Vlad Orlovsky]

you can read this in your spare time
http://www.sielc.com/upload/file/pdf/SI ... e_2005.pdf

[DJ]

Thanks for the replies.

Interesting stationary phase design, Vlad. Have you characterized selectivity with LFER?

[Vlad Orlovsky]

We did not use LFER, since our columns provide much more sellectivity and designed to separate ionic, non-ionic compounds by combination of reversed-phase and ion-exchange mechanisms. You are doing 2D approach on one column.

[DJ]

Vlad:

In principle, is ODS not also a cation exchange mixed-mode column? I'm having trouble understanding how electrostatic interactions from residual silanols on ODS is different from electrostatic interactions with embedded sulfonate or COO- group?

[Vlad Orlovsky]

ODS columns were indeed first mixed-mode columns, particularly non-endcapped one, which left silanols exposed to cation-exchange interaction. Majority of the silanols have pKa of around 5 with some groups having lower pKa. This means that you need to be at pH above 6 to have any controllable ion-exchange interaction. Amount of silanols and pKa of them make them a poor source of ion-exchange which can be easily overloaded. This will cause tailing for basic analyte and loss of retention. In case of carboxylic acid on the surface pKa is much lower, and also the number of these sites is much higher. This allows you to have a wider range of mobile phases, better retention control and loadability.
Here is an article on evolution of mixed-mode and also on overloading of silanols:
www.chromatographytoday.com/article_read/1070/

A study of retention and overloading of basic compounds with mixed-mode reversed-phase/cation-exchange columns in high performance liquid chromatography Nicola H. Davies, Melvin R. Euerby and David V. McCalley 2007 Journal of Chromatography A, Volume 1138, Issues 1-2, 5 January 2007, Pages 65-72

Let me know of you have more questions.