Any explanation for "reverse fronting" peak shape?

[HW Mueller]

Well, whatever, the multiple type of interactions of these columns cause them to be highly responsive to small mobile phase changes. The right conditions, if they exist at all, are therefore not so easy to find, as I am currently experiencing. I might have missed it, but what happens if you use toluene in the mobile phase, or cyclohexane, or a combination of these?

[Tony]

I think I mentioned in one of my previous posts that toluene is extremely effective in eluting my olefins. With 25% EtOH/ 75% toluene, all are eluted in < 3min. However, substituting DCM or hexane at the same concentrations leads to elution >50 min and not at all in <60 min, respectively. I've yet to do more development work with lower concentrations of toluene to increase retention and examine peak shapes. I haven't yet tried cyclohexane - but I think an aromatic ring might be important, as the furan THF is also a weak eluting agent, at least weaker than DCM in my trials.

[HW Mueller]

Ok, so we pretty much got the proof that the column better have the right solvent. I suggested cyclohexane as it has a very slight olefinic character, it would be interesting to see whether this is borne out here, either alone or with a little toluene in it?

[chromatographer1]

This is an interesting post and thread. Since this forum is for discussion and not just the communication of scientific facts, I chose to blab a bit right now.

First, let me say that I am no expert on graphitic or glassy carbon liquid-solid phase chromatographical interactions. So much of what I have to say is antidotal in nature. (aka: reader, beware!)

Graphitic carbon is a very interesting material, as is glassy carbon for the selectivity of molecular interactions both in gas and liquid phase chromatography.

There is a competition for active sites on the surface of graphitic carbon. The number of H-C bonds and a decrease of H-C bonds (C=C bonds) between similar molecules will give them different retention if the competing mobile phase does not interact enough to affect the slight difference in selectivity to the graphitic surface. An olefinic hydrocarbon with a single double bond will elute faster than a similarly structured saturated hydrocarbon molecule, yet an olefin with two conjugated double bonds will be retained more strongly than a saturated hydrocarbon of similar structure. Pi electrons in an aromatic ring (benzene or toluene) or densely available in a molecule like tetrachloromethane or chloroform will be strongly attracted to these surface active sites and will more strongly compete for these sites than H-C or H-O-C bonds theoretically.

I am not well informed on the intra-graphitic interactions but I would assume that the size of the pores within the graphite particle would give a great contribution to the nature of the availability and the strength of attraction of active sites of the surface.

I suspect that the asymmetrical peak shape described by the original post may be an effect of 'overload' of the analyte for the number of available strongly attractive sites. ( OR, it could be the nature of the porosity of the graphite given the size of the pores and the size of the analyte. Are the pores continuous through the particle or are the pores just deep 'holes'? Are we talking then about a mass transfer issue? )

The toluene more strongly competes for these few sites and for the active sites more moderate in strength. Thus the analyte elutes quickly when toluene is in the mobile phase. Chloroform and carbon tetrachloride also might interact in a similar manner. I suspect the 'shape' of the flat ring also contributes to its effectiveness in displacing less strongly attracted analytes to the graphitic surface.

Is any of the above true? I don't know, but thanks for listening. It was fun to blab a little and I hope I did not bore anyone fatally. I would like to take a course or read a good book about this chemistry. Does anyone have any suggestions?

[HW Mueller]

Well, as mentioned above there was an earlier, very good discussion on this.
My guess is that the pore size will play a minor role for most substances run on Hypercarb.