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.
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.
[2][3][4] The Kovats index applies to organic compounds.
The method interpolates peaks between bracketing n-alkanes.
For isothermal gas chromatography, the Kovats index is given by the equation: where the variables used are: The Kovats index also applies to packed columns with an equivalent equation: Compounds elute in the carrier gas phase only.
Compounds solved in the stationary phase stay put.
in the stationary liquid polymer phase is called the capacity factor
: where the variables used are: Capillary tubes with uniform coatings have this phase ratio β: Capillary inner diameter
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.
is introduced with average gas velocity
but shorter, following temperature programming experience.
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
or ß or carrier gas velocity
, which compares favorable to retention time
Isothermal Kovats index is based on solubility
Isothermal Kovats indices of hydrocarbon were measured by Axel Lubeck and Donald Sutton.
[5] IUPAC defines the temperature programmed chromatography Kovats index equation: NOTE: TPGC index does depend on temperature program, gas velocity and the column used !
ASTM method D6730 defines the temperature programmed chromatography Kovats index equation: Measured Kovats retention index values can be found in ASTM method D 6730 databases.
An extensive Kovats index database is compiled by NIST [1].
The equations produce significant different Kovats indices.
Faster GC methods have shorter times but Kovats indexes of the compounds may be conserved if proper method translation is applied.
Temperatures of the temperature program stay the same, but ramps and times change when using a smaller column or faster carrier gas.
If column dimensions Length×diameter×film are divided by 2 and gas velocity is doubled by using H2 in place of Helium, the hold times must be divided by 4 and the ramps must be multiplied by 4 to keep the same index and the same retention temperature for the same compound analyzed.
Method translation rules are incorporated in some chromatography data systems.