Chromatography Forum

This mass transfer explanation is interesting. Why is this so? Less porosity? I always thought that specific area diffs were responsible.

hi rhaefe

i tried to google for any reference citing your info...sadly i could not come up with any..could you tell me the paper which describes this??

Also I just read that PS-DVB columns behave equal to C18 and C8 columns..can any one tell me the reason for this?? Are these support materials hydrophobic similar to c18 columns? What about the retention mechanism..is it same as other traditional columns???

(parden me if i am sounding too novice)

The reason for the difficulties of polymer packings with small molecules are the micropores in the polymer network, which do not exist in inorganic packings. Small molecules get into these pores, but the diffusion is slow. Macromolecules are not bothered by this problem.

Also I just read that PS-DVB columns behave equal to C18 and C8 columns..can any one tell me the reason for this??

I don't know about "equal", but certainly "similar retention". PS-DVB consists essentially of aromatic rings linked together. Think of the surface as being chemically similar to that of a phenyl bonded phase (which would have similar retention to a C18 or C8 ).

Are these support materials hydrophobic similar to c18 columns?

Yes, per the above comment, you can think of them as being similar to a phenyl bonded phase.

What about the retention mechanism..is it same as other traditional columns???

The "bulk" retention mechanism is, indeed, hydrophobicity, as in any reversed-phase separation. Secondary mechanisms are different in the absense of silanols, so selectivity would be different, (but then, selectivity varies widely among the silica-based materials).

There is a short section in the following publication that says essentially the same what I stated earlier:

U. D. Neue, "Silica Gel and its Derivatization for Liquid Chromatography", in "Encyclopedia of Analytical Chemistry", R. A. Meyers, Ed., John Wiley & Sons, Ltd., Chichester (2000), 11450-11472

Hi

I was checking up the available polymer based columns and found that there are two support materials commonly found – (1) polystyrenedivinylbenzene (PSDVB), and (2) polymethacrylate (this is usually found functionalized with C18 and C8 groups), as in Waters YMC columns

Can any one tell what is the difference between both phases..(Waters claim that polymethacrylate is comparatively better than PSDVB)??

P.S.:- are there more chemistry available ???

Thank you

rick1112:

sorry for the late response.
As Uwe pointed out polymer based columns (polystyrene-co-divinylbenzene or PS-DVB) have a significant amount of micro pores. These micro pores form during the suspension polymerization process and are accessible to small molecules. Diffusion in those pores is slow which leads to to the restricted mass transfer rate between phases and to peak tailing. As I pointed out earlier higher separation temperatures can compensate for that but only to a certain degree.
Another effect distinguishes polymeric from silica based phases: the polymeric network can be solvated by organic solvents (swelling phenomenon of polymer particles mentioned earlier). This happens in even highly crosslinked polymers. What this means is that within the polymer network there is usually a certain amount of organic solvent present. This "trapped" solvent takes a finite amount of time to be released from the polymer. Slightly longer re-equilibration times after gradient runs are therefor necessary than compared with silica columns. Again, higher separation temperatures are speeding up equilibration.

PS-DVB phases are true reversed phases and can be used just like C-18 silica phases. Selectivity will be of course different. Surface modifications are possible: C4, C8 and C18 modified PS-DVB have been made in the past. At one point Hamilton had an aminopropyl PS-DVB column available. And of course PS-DVB polymers are the basis for a whole array of ion-exchange columns (anion and cation exchange). Any organic reaction that will utilize the aromatic rings in the polymer network can be used for potential surface modifications.

I will have to dig up some literature references.

One can even find styrene-divinylbenzene columns with a C18 derivatization...

I don't think that there are giant differences between one polymer type and another polymer type. Note that YMC is selling many different types of silica-based packings, but only one or two polymer-based packings.

Another weakness of polymer-based packings compared to silica-based packings is the lack of mechanical strength. Many can be used only at lower pressures. The better ones cover the HPLC range, say to about 250 bar or 4000 PSI, but none is strong enough for the UPLC range.

Officially Hamilton's PRP-1 column are pressure stable up to 5000psi (=345bar). I say officially because this limit seems to coincide with the pressure cut-off of traditional HPLC systems (which used to be around 400bar).
I personally have packed RP PS-DVB columns (not PRP-1 columns although I might give it a try) in excess of 10,000psi (700bar) and the polymer held up without any problems and the columns performed just fine.

We need to be careful about the interpretation of "something can be packed at xxx pressure". I think that we all understand that there is a difference between a piece of rubber and a rock. Polymer particles are like rubber. Silica particles are like rock.

Naturally polymers will be softer than silica. My point was that high pressures (even ultra high pressures) will not necessarily destroy the actual polymer beads or the packing itself.
The right polymer columns can be used without problems up to 350-400bars and some of them can go even higher without loosing performance.

Packing the polymer particles under constant conditions might be one thing, working with them under gradient conditions at UPLC pressures might be another thing. If you can not actually use them at higher pressures there is not much use to it. It would be interesting to see what is their lifetime in UPLC conditions...

What about organic polymer monolithic columns? You were talking about differences between particulate polymer and silica columns.
It is known that polymer monoliths have the same disadvantages as particulate ones: swelling and shrinking, lack of mechanical stability, poorer reproducibility compared to the silica columns (slower mass transfer between mobile and stationary phase), worse peak shapes for small molecules (due to the presence of extra pores that form during polymerization process) etc.
But polymer monolithic columns run under lower pressures, so the high pressure issues of the particulate polymer columns aren't present here (to greater or lesser extent).

Do you think that maybe polymer monoliths have better properties then their particulate "cousins"?

Kostas:
I never proposed using polymer based columns under UPLC conditions or stated that they even can be used successfully under such conditions. I do not have the data to support such a claim and I also think that the earlier mentioned restricted mass transfer in polymeric columns is the biggest obstacle in using them for ultra fast separations of small molecules. The point is that polymeric columns can easily withstand regular HPLC back pressures (i.e. 350-400bar) without any problems, even under gradient conditions. They are chemically and mechanically stable.
As for packing conditions: I don't want to get into details here but packing conditions (packing parameters) are tightly controlled but not necessarily constant throughout the packing process.

This discussion has prompted me to think a bit theory. Since the atoms C and H are rather harder than Si and O one would think that it should be theoretically possible to create a "polymer" column that is more like a rock (diamond) than a silica column. It would have to be a new type of polymer? Maybe based on the adamantane structure or bucky balls?? What about carbon itself? The Thermo Hypercarb is more stable (mechanically, also) than polymer columns (if I read correctly, personally I have seen no problem due to pressure. etc.), but probably not up to par with silica columns?