By Søren J on Monday, July 12, 2004 - 08:10 am:

How are graphite columns compared to "normal" C18 columns with regard to retention times, resolution and using 100% aqueous mobil phases?

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By Mark on Monday, July 12, 2004 - 11:03 am:

Soren,
Since graphite is a totally different column than silica based C-18 the retention times and resolutions of analytes are totally compound dependant, you have some other separation mechanisms that might be operating on graphite you would not see on the C-18. Ron Majors had a good article "The Role of Porour Graphitic Carbon in HPLC" in the January 2000 issue of LC-GC magazine in the Column Watch section. I have never used a porous graphitic carbon column with 100% water so don't know the answer to that one.

Regards,
Mark

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By Chris Pohl on Monday, July 12, 2004 - 03:44 pm:

Soren,

The reference above provides a good summary of the differences between porous graphite and C18 materials but in broad terms two main differences may be expected: overall, more retention of hydrophobic compounds is observed with porous graphite when compared to C18 and selectivity for polar and aromatic compounds can be significantly different.

A couple of other things to watch out for: first, I would expect (I haven't seen anything one way or the other on this in the literature) that stationary phase dewetting in 100% aqueous eluent should be significantly more problematic one porous graphite. Second, several reports have shown evidence of anion exchange capacity (anionic species reportedly exhibit significant retention unless the mobile phase contains an electrolyte and the effect is ionic strength dependent). Some articles imply that the retention of anions on porous graphite is due to some mechanism other than anion exchange but all the evidence is consistent with an anion exchange mechanism. So, just as with silica one must always be concerned with the properties of residual silanols when designing the separation, with porous graphite one must be concerned with anion exchange properties when developing a new method.