Retention index in temperature-programmed gas chromatography

A persistent feature in the development of temperature-programmed gas chromatography (TPGC) has been the search of a retention index IT, defined as a relationship between the retention of the analyte and two members of an homologous series enclosing it. The motivation is to find for TPGC a parameter IT resembling the properties of the isothermal Kováts index I. The latter is a relative thermodynamic parameter which provides basic information on solute–solvent interactions, independently from the chromatographic flow conditions, column’s dimensions and phase ratios. Several years ago, Rijks and coworkers pointed out that the isothermal Kováts index I and the TPGC index IT are incommensurable. The implications from their remark have not received sufficient attention. As a consequence, in many recent publications IT is still presented as a general useful parameter to be reported, specifically with reference to its correlation to solutes structural properties and their partition coefficients, analogous to the applicability of I. A renewed discussion on IT is proposed, with the aim of divulging the unsolvable difficulties for obtaining relevant information from this that could be shared between laboratories. The influence of the column inlet pressure pi and the ratio L/dc on the reproducibility of IT is utilized as an example to illustrate and discuss the basic concepts. Conditions were selected involving capillary gas chromatography of solutes presenting a retention reversal with the bracketing n-alkanes. Several important aspects of IT were illustrated under these conditions. Since basic information of general applicability cannot be derived from IT, it is concluded that only a compilation of thermodynamic parameters can reliably be shared by chromatographers.