Carbon Load vs. Bonding Density

Most of the column manufacturers provide the carbon load value for their reversed-phase adsorbents. Some of them, but not all, also give you the value of bonding density. Which one should be used in the column selection process?

Intuitively, the higher the carbon load, the higher the hydrophobicity of the adsorbent surface, but if we consider the example in the following table we find out that it is not true.

Adsorbent Surface area Carbon Load Bonding Density
  [m2/g] [% w/w] C18 [mMole/m2]
Nonporous silica 4 0.24 4.0
Porous Silica 300 12 2.3

First adsorbent on the table above has nonporous particles with low surface area. Percent of the carbon by weight is low, 0.24, since nonporous particles are heavy and not much could be bonded on their outer surface. On the other hand, porous material could have much more carbon load. A totally porous particle does not have much of the silica, mainly porous space with high surface area. For the second adsorbent we have almost fifty times higher carbon load. Is it good in terms of its HPLC behavior? Actually we can not say anything unless we recalculate carbon load into the amount of C18 chains per gram of the adsorbent and relate this value to the surface area, which give us bonding density value. This parameter (bonding density) actually shows us how well silica surface is shielded. In our example the first adsorbent with just 0.24 % of carbon shows 4.0 mMole/m2 bonding density, which is almost the highest possible. The second adsorbent has only 2.3 mMole/m2, which means that only about 70% of its surface actually shielded, and we may expect a significant tailing for polar components on the column packed with it.

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