Chromatography Forum

Hi,

I am trying to compare between 6 columns all with C-18 solid phase. I got different resolution values for 3 PAH isomers. I noticed that the best resolution achieved by the columns with the highest carbon load and surface area but low pore sizes.

I read in this forum that carbon load only, is not the best way to compare between different C18 columns and it is better to use Carbon Surface Coverage. However, I could not find this value for most of the columns I used.

My Questions:

Is there any equation to calculate Carbon Surface Coverage based on the other column variables?

How can we relate resolution to the Carbon Surface Coverage?

Thank you in advance.

If you know the %C, and you know the chemistry of your bonded phase (C8, C8, etc., mono-, di-, or trifunctional, identity of the side groups), then you can convert to micromoles of bonded phase per gram. If you know the specific surface area (square meters per gram), then you can simply use the ratio to calculate the surface coverage (micromoles per square meter).

Correlating that with resolution is entirely empirical; it will depend on the selectivity of the columns for your specific sample compounds. Bonded phase density is only one of a number of parameters which will affect that.

Thank you Tom for this reply.

However, according to publications by Lane Sander, the surface coerage is the important factor for this type of selectivity.

All right, Uwe, I'll bite:

1. I was reading the original most more broadly ("How can we relate resolution to the Carbon Surface Coverage" not "How can we relate resolution of these compounds to the Carbon Surface Coverage").

2. You make a good point that surface coverage is an important factor, it's not the only factor.

There is this NIST test kit of PAH's for testing the nature of your stationary phase, NIST 869. It contains three PAH's one of them is benzo-a-pyrene. The relative retention (alpha) of the BaP to the other two correlates perfectly with surface coverage, at least in the study that Lane published. I think it was published in J. Chrom. 656 (1993), 335.

I was impressed with this correlation, which is why I remember it.

Thanks for the reference! I wasn't familiar with the details, so I'll have to dredge it out.

These kind of studies should be forbidden!!!

They takes the beutiful mysteries behind HPLC!