Compounds elute in the carrier gas phase only. Compounds solved in the stationary phase stay put. The ratio of gas time  and residence time  in the stationary liquid polymer phase is called the capacity factor :
where the variables used are:
*  gas constant (8.314J/mole/k)
*  temperature [k] 
*  solubility of compound i in polymer stationary phase [mole/m]
*  vapor pressure of pure liquid i [Pa]
Capillary tubes with uniform coatings have this phase ratio β:
Capillary inner diameter  is well defined but film thickness  reduces by bleed and thermal breakdown that occur after column heating over time, depending on chemical bonding to the silica glass wall and polymer cross-linking of the stationary phase. 
Above capacity factor  can be expressed explicit for retention time: 
Retention time  is shorter by reduced  over column life time. Column length  is introduced with average gas velocity : 
 and temperature  have a direct relation with . However, warmer columns ↑ do not have longer  but shorter, following temperature programming experience. Pure liquid vapor pressure  rises exponentially with  so that we do get shorter  warming the column ↑. Solubility of compounds  in the stationary phase may rise or fall with , but not exponentially.  is referred to as selectivity or polarity by gas chromatographers today. 
Isothermal Kovats index in terms of the physical properties becomes:
Isothermal Kovats index is independent of , any GC dimension  or ß or carrier gas velocity , which compares favorable to retention time . Isothermal Kovats index is based on solubility  and vapor pressure  of compound i and n-Alkanes ().  dependence depends on the compound compared to the n-alkanes. Kovats index of n-alkanes  is independent of .
Isothermal Kovats indices of hydrocarbon were measured by Axel Lubeck and Donald Sutton.