4.2.3 Retention Index and Relative Interactions with Stationary Phase

The RI shows the relative strength of the interaction of a compound with the stationary phase. The RI scale is defined using the straight chain hydrocarbons, which are non-polar. A polar compound is retained more strongly by a polar stationary phase (and less than non-polar stationary phases), so its RI is larger than one predicts based solely on the number of carbon atoms. For example, cocaine has 17 carbon atoms, yet has a RI of approximately 2600 with a particular slightly polar stationary phase.

• The straight chain hydrocarbons are non-polar, so the only intermolecular interactions that are possible are dispersion forces, dipole-induced dipole forces and ion-induced dipole forces. The magnitude of the dispersion force increases with molecular volume, which is roughly equivalent to saying the dispersion force increases with molar mass.
• Cocaine (for example) has 17 carbon atoms, yet the RI is 2660 with a phase that is slightly polar. This suggests that the interaction of cocaine with this phase is stronger than the interaction of a non-polar molecule with the polar phase; cocaine is interacting with the phase with an interaction stronger than the dipole-induced dipole interaction expected for an equivalent sized non-polar molecule.
• As an exercise, the Intern should look up the structure of cocaine and briefly describe why the structure is consistent with the molecular interaction with a polar phase being stronger than a dipole-induced dipole interaction. What might the principal interaction be?

No matter what the stationary phase is, the RI is defined to be 1000 for n-decane, etc. The retention indices of other compounds vary for different stationary phases. However, for a particular stationary phase, the RI is constant, so this allows the comparison of retention data for different instruments, different run conditions, etc.