NUCLEODUR^® – C18 Isis

By the use of specific C18 silanes and appropriate polymeric bonding technologies, a dense shield of alkyl chains protects the subjacent silica matrix. Elemental analysis of NUCLEODUR C18 Isis shows a carbon load of 20%.

The target crosslinking of the C18 chains on the surface enables the separation of compounds with similar molecular structure but different stereochemical properties. The technical term for this feature is steric selectivity.

The chromatograms on the right reveal the improved resolution for positional isomers in a test mixture of aromatic compounds on NUCLEODUR C18 Isis (1) in direct comparison with monomerically coated (2) and polar endcapped (3) C18 columns.

Sander and Wise [LCGC 8 (1990) 378-390] proposed a model for the retention of aromatic compounds based on molecular shape, which is referred to as the “Slot Model”. This model pictures the bonded C18 phase on the silica surface with slots that the analytes have to penetrate during retention.

Planar molecules are able to penetrate these slots deeper than non-planar molecules of similar molecular weight and length-to-breadth ratio. Thus triphenylene is longer retained than o-terphenyl.

The separation of o-terphenyl and triphenylene is a concrete example to evaluate the selectivity potential of a reversed-phase column in terms of the different shapes of two molecules. The phenyl rings of o-terphenyl are twisted out of the plane while triphenylene has a planar geometry.

The separation factor (a value) is a measure for the steric selectivity. As is shown in the following chromatograms the a value is considerably larger on NUCLEODUR C18 Isis compared to a conventional C18 column.