Chromatography Forum

I cannot recall the citation or journal, but there are reports of researchers converting a C18 into a mixed-mode RP/SAX by conditioning a plain C18 column with excess phase transfer catalyst (quaternary amine with at least one long-chain R-group.). Supposedly, once coated, the PTC is embedded and there to stay. Other publications initially coat a C18 with the PTC, but then also include low mM amounts as a mobile phase additive.

I wonder, could one not do an analogous treatment, but with SDS (or a very hydrophobic anionic detergent, perhaps, a phospholipid) to convert a C18 RP column into RP/SCX mixed mode?

It's done all the time under the name "ion pair chromatography".

To my mind, the problem with just using the residual ion-pair reagent in that fashion is that you give up the advantage of being able to control the ion-exchange capcity the way you can with conventional ion-pair (by adjusting concentration or changing chain length) or mixed mode columns (which often use weak exchangers that allow you to exploit pH).

In addition, there is some question about how much irreversible binding you would get with an anionic surfactant. The cationics are arguably bound not only by their hydrophobicity, but also by cation exchange to ionized residual silanols.

why in the world you want to do that instead of using mixed-mode column?

why in the world you want to do that instead of using mixed-mode column?

We have tons of old C18 columns.... and we're not exactly feasting on caviar and champagne.. as a matter of fact, I'm nearly one can of drain cleaner and a bottle of cheap vodka away from having to make my own ether.

then you need to find another job. This is not the way to treat a valuable employee. The most expensive parts in any lab are people.

DJ, I believe you're in academia, right? In which case the economics are a bit different than they are in industry because you have (or are) indentured servants (= grad students).

Economics aside, there's no harm in trying, but I'm not hopeful that you would get reproducible (or indeed any useful) results without having surfactant in the mobile phase -- in which case you are doing ion-pair chromatography.

Hi,

If you're looking for a mixed mode column, you may want to give the Scherzo family a try (from Imtakt)! It has a novel stationary phase with anion, cation, and C18 ligands (so you would not need to convert a C18 column to a mixed mode column....it is already done for you)! The unique feature of this column is that it has all three types of ligands, anion, cation, and C18. You can vary the retention by modifying the pH and/or the salt concentration. Please see link below for more details. If you would like more information, feel free to email me at bverma@imtaktusa.com. Thanks!

http://www.imtaktusa.com/site_media/fil ... family.pdf

DJ, I sympathise with your economic situation. You are not alone. I am not yet back to the days of washing vials, or poking out the septa from lids so I can put new septa in...

bverma,
Perhaps you don't know history of mixed-mode. There is nothing novel about Scherzo columns, Imtakt is using an old approach of physically mixing at least two types of silica gel, which is considered a first generation of mixed-mode columns known for 30 years (Duet column from "Old" Thermo known for decades). Dionex (now Thermo Fisher) and us (SIELC) are using second and third generation mixed-mode columns which incorporate a single particle/ligand design which carries two or three functionalities (reversed-phase, cation- and anion-exchange). The first mixed-mode column (I call it generation Zero) was non-endcapped RP silica where cation-exchange came from residual silanols.

That's quite interesting. So what do you do with a mixed mode column that contains positive and negative charges, that you couldn't do with a "regular" mixed mode column that has just one type of charge.

If you have hydrophilic counter-ions you are not going to retain both of them is only one ion-exchange mechanism is available. So if for example you need to analyze sodium, potassium and chloride dual-mode column is not going to retain cation and anion. There are other cases when tri-modal column will work much better and will give you a better range of compounds which can be analyzed. Multi-mode columns were developed to expand the range of compounds analyzed within one run (drugs and corresponding counter-ions)
http://www.sielc.com/upload/file/pdf/SI ... y_2009.pdf

Imtakt has an application that separates both and acid and a base simultaneously, it is below:

http://www.imtaktusa.com/site_media/fil ... I690E_.pdf

For more information, please contact me at bverma@imtaktusa.com. Thanks!

Imtakt has an application that separates both and acid and a base simultaneously, it is below:

http://www.imtaktusa.com/site_media/fil ... I690E_.pdf

For more information, please contact me at bverma@imtaktusa.com. Thanks!

This is not a very good example, because both compounds in your application can be retained by reversed-phase mechanism and in application you showing demostrates this. Any C18 column will retein and separate iburpophen and chrlorpheiramine. True benefits of mixed-mode are for compounds which are not retained and separated by simply one mechanism

OK I am going to belabor this a bit, for my own education.

Just so I understand, why is the second generation better than the first generation: either way you have both reversed phase and ion exchange.

And what exactly is third generation.

Is there is a review article or book chapter that you can point us to.

Much Thanks

Here is some opinion in the article we wrote in 2011 about "Evolution of Mixed-Mode Chromatography"
http://www.chromatographytoday.com/article_read/1070/

last year we also got an offer to write a book on Mixed-Mode Chromatography from Wiley, but unfortunately did not come to a deal. we realized that it is incredible amount of work to write 300 pages...and we need to be non-bias, so the book never materialized.