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Retention indexes are determined using the following general principle. When a mixture containing compounds belonging to a homologous series of n‐alkanes is injected onto a column maintained in isothermal mode, the resulting chromatogram is such that the logarithm of the adjusted retention times t′_R(n) increases linearly with the number n of carbon atoms present in the corresponding n‐alkane (Figure 2.17).

Figure 2.17 Kovats straight line graph. Left, isothermal chromatogram for a series of five n‐alkanes (C₁₀−C₁₄). Right, Kovats’ relationship for the chosen stationary phase and for the predetermined analytical conditions.

On a graphical representation, the carbon number n versus log t’_R(n) usually yields a series of well‐aligned points:

(2.3)

The adjusted retention time t’_R(n) corresponds to the retention time t_R of an alkane having n atoms of carbon, minus the dead time t_M; a and b are numerical coefficients. The slope of the graph obtained depends on the overall performance of the column and the operating conditions of the chromatograph.

This expression follows on from another linear relation seen in thermodynamics linking the variation in free energy and the equilibrium constant K (∆G = −RT ln K), for a homologous family of compounds in which each term differs from the preceding one by an additional CH₂ unit. Since K = kβ, ; thus log will increase with ln K for the homologous family: ln .