Generalization of the gas chromatographic retention index system

Abstract

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 Kováts retention index ( n-paraffin scale) of a substance i, I p ( i), is I p ( i) = 100[ n( i) + G(X i / G(CH 2)], where n( i) is the carbon number of the substance and G(X i ) and G(CH 2) are the standard molar is the carbon number of the functional group X i of the substance and of a CH 2 group in the hydrocarbon chain of the substance. The general retention index, I a ( i), is I a ( i) = 100{ n( i) + [ G(X l ) - G(X a ]/ G(CH 2)} where a signifies a reference homologous series of compounds with functional group X a , G(X a ) being the respective sorption Gibbs free energy. The quantities I a ( i) and I p ( i) are related to each other by the equation I a ( i) = I p ( i) - I p ( a) + 100 n( a), where I p ( a) is the paraffin-scale retention index of a reference compound of type a and n( a) is its carbon number. The difference I p ( i) - I a ( i) on a given sorbent is equal to I p (X a ), i.e., it is not a characteristic of the compound i. For measurements on different sorbents, differences in retention index on sorbents A and B are given by the equation I A a ( i) - I B a ( i) = I A p ( i) - I B p ( i)-[ I A p ( a) - I B p ( a)].