It is possible to express the retention index of a given substance, chromatographed on a given sorbent, in different ways by employing different series of compounds to define the reference retention scale. Thermodynamic analysis of the concept of retention index shows that the conventional Kov&s retention index (nparaffin scale) of a substance i, JJi), is I,
where n(i) is the carbon number of the substance and G(X,) and G(CH2) are the standard molar sorption Gibbs free energies of the functional group X, of the substance and of a CH2 group in the hydrocarbon chain of the substance. The general retention index, I,(i), is I,(i) = lOO{n(i) + [G(X,) -G(X,)]/G(CH,)} where a signifies a reference homologous series of compounds with functional group X,, G(X,) being the respective sorption Gibbs free energy. The quantities I,(i) and I,,(i) are related to each other by the equation I,(i) = I,,(i) -1,(a) + 100 n(a), where I,,(a) is the paraffin-scale retention index of a reference compound of type a and n(a) is its carbon number. The difference I,(i) -&(,(i) on a given sorbent is equal to 1,(X,), i.e., it is not a characteristic of the compot:nd i. For measurements on different sorbents, differences in retention index on s&bents A and J3 are given by the equation I:(i) -/t(i) = I:(i) -I:(i)-V;(a)qxdl.