Just a general question regarding graphite columns.

I know that reverse phase columns made of bonded silica have varying stability issues at pH extremes.

I know that polymeric phases have mechanical liabilitys and I've experienced that they don't give the resoultion and specificity that bonded silica does.

I could talk to a variety of graphite column vendors about my specific separation and evaluate some columns, but I'd just like to know a little bit about graphite.

How come no one ever seems to mention them? Do they make graphite with C18 bonded to it?

Do they behave so similarly to silica or polymeric so as to make no difference?

Do they behave exactly at the middle of all benefits and shortcomings so they're just not as useful as replacements?

Do they produce very different separations?

Too expensive? Something else?

Our principle use is non-polar derivitized biomolecules. We use analytical HPLC and prep scale. And I do mean prep -- we use 10 cm diameter columns, polymeric

Actually, that was about seven questions. I'll stick my neck out and give some opinions (you will probably also get differing ones from others!)

1. In broad terms, the pH issues with "conventional" silica-based bonded phase columns boil down to:
- loss of bonded phase at low pH (cleavage of the silyl-ether bonds that hold the phase on).
- dissolution of silica at high pH (silica gel is made by dropping the pH of a sodium silicate solution; this is reversible -- take the pH up and the stuff wants to go back in to solution).
Note that there are ways to counteract both problems, and that there are now a fair number of silica-based reversed-phase columns with extended pH ranges.

2. Polymer-based columns (e.g., polystyrene-divinylbenzene copolymers) are inherently stable over the entire pH range. The mechanical stability and diffusion characteristics are not as good as those of silica , so they tend to provide somewhat lower efficiency, which is why silica-based columns still dominate the market (my opinion here, others may disagree). Selectivity is *different* from that of silica-based columns because there are no residual silanol groups to help (or interfere) with retention. Differences in selectivity can cut either way, depending on your specific sample.

3. There is not much point in attaching C18 to graphite or polystyrene. The only reason it's done on silica is to put a hydrophobic surface on a support material that has good diffusion characeristics. Polystyrene and graphite are hydrphobic to begin with, so derivatization to C18 is uneccessary.

4. Graphite columns differ in a fundamental respect from both polystyrene and bonded-phase silica in that the hydrophobic surface of graphite is crystalline (essentially, rigid instead of squishy). This means that graphite can provide significantly better selectivity for isomer separations than other reversed-phase media (remember that for complex mixtures, this can be a curse as easily as a blessing). In essence, graphite combines an isomer selectivity somewhat like that of traditional adsorbents like bare silica or alumina with a reversed-phase retention mechanism.

5. By reputation, graphite columns are more fragile than silica-based or polymeric columns. This is out of my area of expertise, but crystalline graphite is basically planar, with very little holding adjacent planes together. (If anyone wants to comment on or correct this perception, I'd be grateful!).

6. Graphite columns are more expensive than silica-based or polymeric column. I don't know whether this is a manufacturing-cost or production efficiency (they don't make enough to get economy of scale, so the price is high, so they don't sell many, so they don't make enough to get economy of scale, . . .) issue. I rather expect the former.

7. My $0.02 worth: I've recommended graphite for separations (mainly isomer separations) where silica-based or polymeric columns don't have the requisite selectivity. Due to cost and fragility issues, I wouldn't turn to graphite for a prep separation unless it were the only way to do the separation.

As they say in car ads, "Your mileage may vary!"

-- Tom Jupille / LC Resources Inc.

Thanks Tom -- cost and fragility sum it up for me. The problem of better isomer selectivity hurting the separation of a complex mixture is also good to know

Does anybody make columns from diamond particles? That would get around graphite's fragility problems.