Silica based adsorbents modified with trimethylchlorosilane (C1) and buthyldimethylchlorosilane (C4) have a few applications in HPLC, mainly for protein separation or purification. These adsorbents show significant polar interactions, although they do not have specific interactions caused by acidic silanols.
Octyl (C8) and octadecyl (C18) modified adsorbents are the most popular ones. Almost 80% of all HPLC separations have been developed with these adsorbents.
Table below shows the main parameters of some commercial adsorbents
|Name||Vpore||Dpore||Surf area||Carbon. loading||BP density|
|Hypersil C4 (3)||0.6||300||50||2.0||4.80|
|Supelcosil C8 (3)||0.6||100||170||8.5||-|
|Hypersil C18 (3)||0.7||120||170||10.0||2.84|
Propylphenylsilane ligands attached to the silica gel show weak dipole - induced dipole interactions with polar analytes. Usually this type of bonded phase is used for group separations of complex mixtures. Amino-compounds show some specific interactions with phenyl modified adsorbent.
A cyano modified surface is very slightly polar. Columns with this phase are useful for fast separations of mixtures consistingof very different components. These mixtures might show very broad range of retention times on the usual columns.
Cyano-columns could be used on both normal- and reversed-phase modes of HPLC.
Amino-phase is a weak anion-exchanger. This type of column is mainly used in normal-phase mode, especially for selective retention of aromatic compounds.
Diols are slightly polar adsorbent for normal-phase separations. These are useful for
separation of complex mixtures of compounds with different polarity, and which
usually shows a strong retention on unmodified silica.