General properties of the HPLC adsorbents including that for the reversed-phase separations have been discussed in the "Adsorbent" chapter.
|Adsorbent||Pore size||Pore volume||Surface area||Carbon load||Bonding density|
|IB-Sil C18 (monomeric)||125||0.75||165||11||3.27|
|Ultracarb ODS (20)||90||0.75||370||22||3.53|
|Spherisorb ODS (2)||80||0.5||220||12||2.72|
|Hypersil C18 (ODS)||120||0.7||170||10||2.84|
In the table above we only mentioned the most popular adsorbents and only in C18 form.
Most of them are available in C18, C8, C4, C1, CN and NH2 forms.
Selection of the adsorbent suitable for the particular separation should be based on the parameters mentioned in the table.
Pore size. Usually the pore size value corresponds to the original silica, and after modification it decreases. C18 decreases the pore size on almost 40┼. So, if you are planning to separate relatively big molecules, you better choose an adsorbent with wider pore size.
Pore volume. Not the most important parameter, but together with the surface area and pore size it may indicate the sample capacity and rigidness of the adsorbent. The higher the pore volume and surface area, the more fragile the adsorbent, but, the higher its capacity.
Surface area. This parameter has been discussed intensively in the "adsorbent" chapter. The main point is: the higher the surface area, the longer the retention.
Carbon loading. Percent of carbon atoms by weight per unit of the adsorbrnt weight. Common parameter which could be found in any vendor catalog. Does not have any positive information.
Bonding phase density. Important parameter, which indicates how hydrophobic the
adsorbent is. Theoretical limit for bonding of C18 ligands (based on Van der Waals volume)
is 4.2 Ámole/m2. If an adsorbent has a bonding density more than 3 Ámole/m2,
then in most of the applications it will not show any specific interactions.