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 I-T, 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 I-T resembling the properties of the isothermal Kovats 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 Kovats index I and the TPGC index I-T are incommensurable. The implications from their remark have not received sufficient attention. As a consequence, in many recent publications I-T 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 I-T 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 p(i) and the ratio L/d(c) on the reproducibility of I-T 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 I-T were illustrated under these conditions. Since basic information of general applicability cannot be derived from I-T, it is concluded that only a compilation of thermodynamic parameters can reliably be shared by chromatographers. (C) 1999 Elsevier Science B.V. All rights reserved.