Kovats retention index

In gas chromatography, 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 this concept during the 1950s while performing research into the composition of the essential oils.^[1]

The retention index of a certain chemical compound is its retention time normalised to the retention times of adjacently eluting n-alkanes. While retention times vary with the individual chromatographic system (e.g. with regards to column length, film thickness, diameter, carrier gas velocity and pressure, and void time), the derived retention indices are quite independent of these parameters and allow comparing values measured by different analytical laboratories under varying conditions. Tables of retention indices can help identify components by comparing experimentally found retention indices with known values.^{[2] [3] [4]}

Expression

The method takes advantage of the linear relationship between the values of $\log (t_r')$ and the number of carbon atoms in a molecule. The value of Kovats index is usually represented by I in mathematical expressions. Its applicability is restricted to organic compounds. For isothermal chromatography, the Kovats index is given by the equation

$I = 100 \times \left [ n + \frac{\log (t_{r (\text{unknown})} ') - \log (t_{r (n)} ')}{\log (t_{r (N)} ') - \log (t_{r (n)} ')} \right ]$

Where:

$I =$ Kovats retention index,
$n =$ the number of carbon atoms in the smaller n-alkane,
$N =$ the number of carbon atoms in the larger n-alkane,
$t_r' =$ the adjusted retention time.

For temperature programmed chromatography, the Kovats index is given by the equation

$I = 100 \times \left [ n + \frac{t_{r (\text{unknown})} - t_{r (n)} }{t_{r (N)} - t_{r (n)} } \right ]$

Where:

$I =$ Kovats retention index,
$n =$ the number of carbon atoms in the smaller n-alkane,
$N =$ the number of carbon atoms in the larger n-alkane,
$t_r =$ the retention time.

References

1. Institute of Chemical Sciences and Engineering, KOVATS Ervin, Prof. Hon.
2. Kovats, E. (1958). "Gas-chromatographische Charakterisierung organischer Verbindungen. Teil 1: Retentionsindices aliphatischer Halogenide, Alkohole, Aldehyde und Ketone". Helv. Chim. Acta 41 (7): 1915–32. doi:10.1002/hlca.19580410703.
3. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "retention index, I in column chromatography".
4. Retention index guide