Tuesday, November 15, 2011

Reverse Phase Silica - Sort of!

I'm sure that all the synthetic organic chemists will agree that column chromatography is something we simply couldn't live without. At our Department we use it in all its forms: Automated flash chromatography, Chromatotron, Prep. HPLC (Chiral and RP), Prep. TLC, old skool manual flash and DCVC columns etc. However, in spite of all this we all too frequently end up with stuff that is a major pain to separate. Just last week a colleague of mine, lets call him Bernard, told me he had used deactivated silica for purifying his compound - the only thing that had worked for him! So now you are probably thinking deactivated silica??? Normal lab terminology translates this to treating your silica with some amine base prior to use but this is not what Bernard was talking about. He found this paper (Procedure in the Supp. Info.) where they derivatise silica gel with ethyltrichlorosilane. Cheap reagents and a simple method. I guess the "deactivated" silica ends up being something between regular and reverse phase silica. Bernard simply packed a column with this stuff and put on our automated columning system to get a great result eluting with normal phase solvent mixtures. Excellent news for me and my people. We often end up with very polar compounds that will not come off a regular column or elute in fractions 1-5 on a reverse phase column.
Does anyone have any experience with this type of silica or something similar? Please let us all know by posting a comment. D!

23 comments:

Steve said...

I've used silver nitrate treated silica to separate close running compounds. It's effect is seen most with very apolar compounds that run very quickly. There's a good description of what to do here http://chemknowhow.com/forum/viewtopic.php?t=494

Anonymous said...

Im just intrigued by your coloumn setup/glassware setup.

How does it work?

Anonymous said...

Congrats on your research group!

Daniel Sejer said...

@ Steve: Isn't that the classic column for separating cis-alkenes from trans-alkenes? Thanks for the tip. Will check the link out.
@ Anonymous 1: that's your basic DCVC column. Two posts on the topic elsewhere on this blog.
@ Anonylous 2: Thank you. Still a very tiny research group. Hopefully that will change in the coming years. D!

Steve said...

@Daniel - True, I was separating an alkene. A Feringa type molecular motor - the key part of which is a sterically overcrowded alkene. I only found it necessary to use it for very apolar compounds though - the kind of thing that would elute very quickly in straight hexanes. The silver nitrate treatment seemed to slow them down a lot. It's nice because you can easily make up your own modified TLC plates as well.

Wayne Mascarella said...

I've always understood "activated silica-gel" to be the material referred to in many ASTM analytical methods. It is simply produced by heating silica-gel in a muffle furnace at 130 degC to 180 degC overnight.
[per Bartell and Almy, J Phys Chem, 1932, 36(2), 475 or Cahnmann, Analytical Chem, 1957, 29(9) 1307.]
Just the thing for separating petroleum alkanes and arenes. (Elute first with hexane, then benzene/hexane.)

Daniel Sejer said...

@Wayne: well this is embarrassing. You are right. That's exactly what activated silica is supposed to be. I made a typo. What I am trying to say is "deactivated" silica. Sorry about the mix up. D!

Rang said...

Hey Daniel, I would imagine that the deactivation of silica is due to bonding of C2 silane (ethyl) to silanols. That treatment produces a more hydrophobic surface which allows reduced retention of very polar compounds.

There are other ways to separate polar compounds, though. For example you can do it using Cyano silica, but that stuff is normally very expensive. For very polar compounds you can also use a mixture of ACN/H2O or Acetone/H2O on silica. There's a lot of literature on HILIC(Hydrophilic interaction Chromatography) nowadays.

Cheers,

Rang said...

There's an application note from Teledyne Isco on HILIC purification for flash chromatography. Hope that helps.

http://www.isco.com/WebProductFiles/Applications/101/Application_Notes/AN78_HILIC_Purification_Strategies_for_Flash_Chromatography.pdf

Please note the in HILIC, H2O is the strongest solvent. The solvent strength are the following:

Acetone<ACN<IPA<EtOH<MeOH<H2O

Although both NP and HILIC use underivatised silica as stationary phase, retention mechanism in HILIC is different to that of Normal Phase so HILIC is not the extension of "wahshing the silica using 100% MeOH". Please allow at least 5% H2O in your eluent, otherwise it wouldn't work.

Daniel Sejer said...

@Rang: Thanks for some excellent info about HILIC. This stuff is new to me. Your link to an ISCO page is broken but there is an excellent Wikipedia entry on HILIC:

http://en.wikipedia.org/wiki/Hydrophilic_interaction_chromatography

D!

Rang said...

Hi Daniel, it seems like the original link from Isco is too long and was truncated by the system.

Below is the shortened version, hope that helps:

http://t.cn/S2gNEI

Anonymous said...

Thanks Everyone for the excellent suggestions. I have a mixture of aromatic compounds with a very small Rf difference (~0.05) and have not been able to separate them from a long time. It runs to an Rf of about 0.1 in 0.2% MeOH and 99.8% CHCl3. Which of the methods you mentioned can be useful for the separation?. Any suggestions are welcome. Thanks

Rang said...

Hi Anonymous, I would suggest you try different solvents to optimise the seletivity between your closely eluting compounds. Have you tried Acetone/Toluene or other solvent systems? Change of solvent can have a huge difference in normal phase chromatography. Cheers,

freezedream said...

Thanks for the tip! I once used the more common Et3N-deactivated silica for purifying a primary amine but this looks intriguing and may come in handy.

We have no prep HPLC but use C18 SPE columns instead, though I've not had much success with them.

chidambar kulkarni said...

@Rang : Thanks for your suggestions. But I have tried a lot of solvents and none of them seem to help, even acetone and toluene mixture. I came across a paper in which the silica was modified by 2,4-dinitroanilinopropyl (Analytical Chemistry 1982, 54, 1071-1078) and was shown to be good for the separation of aromatic compounds. Has anybody handled such modified silica. Any comments are welcome.Thanks.

Rang said...

Hi chidambar, a quick look online showed that DNAP silica has many uses in separation of PAHs and PCBs. I have not used this kind of silica myself but chemists who work with PAH or in petroleum sector will be familiar with it.

In HPLC terms, you can probably try Cyano or PFP columns. Have you considered prep HPLC?

chidambar kulkarni said...

@Rang : Thanks. Will try both of DNAP silica and prep HPLC and see which one helps. Thanks for the suggestions.

emunfred said...

I used to buy C2 derivatized silica gel TLC plates. They were a little more polar than C18 reversed phase plates and a little less polar than silica gel. They were good for separation of small, moderately polar molecules. They have been discontinued, I think.

Anonymous said...

Hello!
I like the idea and I would like to give a shot to C2 modified silica gel in separating very polar compounds.
The obvious question that arises is can normal SiO2 TLC plates be modified in the similar way (with ethyltrichlorosilane) to get the good prediction of how will compounds be separated on the column?
Does anybody have the experience with this?
Cheers!

Daniel Sejer said...

@anonymous: That's an excellent question and the major drawback of this method. Bernard tried dipping aluminium backed TLC plates in the reagent. However, after drying they were chipping an flaking all over the place. The way forward could be glass backed TLC plates that generally are less susceptible to this problem. D!

kipchirchir said...

Chidambar:- I worked with unprotected bisphosphonates (VERY POLAR!) and had to get creative with their purification and separation of close spots on silica! I sometimes used three solvent mixes (Dont be afraid to try!) of this type MeOH/EtOH:CHCl3/CH2Cl2:Hexanes/EtOAc. I sometimes had to do two columns to get clean separations! I have heard of people doping in some THF and it made a difference!

Anonymous said...

the silica your refer to is some kind of reverse phase silica, but usually it is grafted with C18 chains. deactivated silica normally refer to pretreatment of the normal silica gel with triethylamine: it gets really hot and the silica once packed in the column is so so white!!! i had good results with this deactivated silica on acid sensitive compound and also organometallic complexes (they were decomposing on alumina, any grade...)

Jimmy Lawrence said...

I tried doing this with TMS silica (TLC grade). I must say it did not go very well. Maybe because the Rf in general goes up with the treated Si.