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The temperature dependence, ΔI/°C, of the retention indices of manybranches paraffins with six to eight carbon atoms was investigated using SF-96 columns, in the temperature range 80-120°. Linear relationships were observed for all the compounds examined, and the calculation of the correct analysis observed for all the compounds examined, and the calculation of the correct analysis temperature for the separation of interesting pairs of isomers was therefore possible. The dependence of ΔI/°C on the skeletal structure of the compounds permitted the determination of characteristic additive terms correlated with structural groups in the molecules, which could be added to give the overall temperature dependence of the index of any isometric compound. The effect of temperature programming on the separation of interesting pairs of isomers was also investigated.

The retention times of branched-chain paraffins were shown to depend on their physical properties, especially molecular volume, when non-polar phases were used for their separation. As the molecular volume depends strongly on structure, the calculation of additive terms, which are characteristic of every structural group in a molecule, can permit the theoretical calculation of the retention indices of any branched isomer. The additive terms can be calculated both by solving a system of equations and by comparison of the retention indices of selected compounds. The general application of the method to retention data taken from the literature is also possible.