Temperature dependence of Kovلts indices in gas chromatography revisited

Temperature dependence of the Kovلts retention index (I) was measured for some aliphatic ketones and aldehydes on a poly(dimethyl siloxane) (HP-1) stationary phase. An interesting minimum (non-linearity) was observed for the I versus isothermal column temperature (T) relationships. A novel empirical model is proposed: I=A+B/T+C ln T, where A, B and C are equation constants and B/C=Tmin. A detailed statistical analysis clearly shows superiority of the extended model (i.e., of this containing the logarithm of the temperature (ln T) term) over the earlier established Antoine-type reciprocal equation. The minimum temperature (and the energy like quantity=RTmin, where R is the gas constant) changes in a systematic manner. The factors effecting the (RTmin) term are as follows: (i) this term decreases with the increase of the molecular mass of the respective oxo compounds; (ii) ketones have higher absolute values of (RTmin) than aldehydes; (iii) branching of the carbon chain lowers the mentioned (RTmin). This enthalpy term is unambiguously bound to the polarity of solutes.