Orthogonality of Analysis

Representation of GC X GC as a two-dimensional separation, with separation mechanisms based of different chemical properties in each dimension. 

The elution of compounds on GC columns is a complex process related to the volatility of the compound, which results from its boiling point, and the chemical interactions between the compound and the stationary phase. These interactions are typically those which arise from polar-polar interactions, dispersion forces, dipole-dipole interactions, and so forth. Collectively, they are described by the term the chemical potential, Ap.°, which derives from the potential for the compound to 

With comprehensive GC, we can now choose a rational set of columns that should be able to tune the separation. If we accept that each column has an approximate isovolatility property at the time when solutes are transferred from one column to the other, then separation on the second column will largely arise due to the selective phase interactions. We need only then select a second column that is able to resolve the compound classes of interest, such as a phase that separates aromatic from aliphatic compounds. If it can also separate normal and isoalkanes from cyclic alkanes, then we should be able to achieve second-dimension resolution of all major classes of compounds in petroleum samples. A useful column set is a low polarity 5 % phenyl polysiloxane first column, coupled to a higher phenyl-substituted polysiloxane, such as a 50 % phenyl-type phase. The latter column has the ability to selectively retain aromatic components. 

The concept of tuning a separation was succinctly summarized by Venkatramani et al. who stated (32)  

A properly tuned comprehensive 2-dimensional gas chromatograph distributes substances in the first dimension according to the strength of their dispersive interactions. .. and in the second dimension according to their specific non-dispersive 

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