In gas chromatography, the Kovats retention index (shorter Kovats index, retention index; plural retention indices) is used to convert retention times into system-independent constants. The index is named after the Hungarian-born Swiss chemist Ervin Kováts, who outlined the concept in the 1950s while performing research into the composition of the essential oils. The retention index of a chemical compound is retention time interpolated between adjacent n-alkanes. While retention times vary with the individual chromatographic system (e.g. with regards to column length, film thickness, diameter and inlet pressure), the derived retention indices are quite independent of these parameters and allow comparing values measured by different analytical laboratories under varying conditions and analysis times from seconds to hours. Tables of retention indices are used to identify peaks by comparing measured retention indices with the tabulated values. Isothermal Kovats retention index The Kovats index applies to organic compounds. The method interpolates peaks between bracketing n-alkanes. The Kovats index of n-alkanes is 100 times their carbon number, e.g. the Kovats index of n-butane is 400. The Kovats index is dimensionless, unlike retention time or retention volume. For isothermal gas chromatography, the Kovats index is given by the equation: where the variables used are: , the Kováts retention index of peak i , the carbon number of n-alkane peak heading peak i , the retention time of compound i, minutes , the air peak, void time in average velocity , minutes The Kovats index also applies to packed columns with an equivalent equation: Kovats index and physical properties 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/m3] 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 .