Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Kovats retention index

Kovats retention index (shorter Kovats index, retention index; plural retention indices) is a concept used in gas chromatography to convert retention times into system-independent constants. The index is named after the Hungarian born Swiss chimist, Ervin Kováts who lied down this retention concept during the 1950s while contributing to the composition analysis of the essential oils.[1]

The retention indices 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, 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.

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

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

Where;

$I=$ Kovats retention index,
$n=$ the number of carbon atoms in the smaller alkane,
$N=$ the number of carbon atoms in the larger alkane,
$t_{r}'=$ the adjusted retention time.

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

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

Where;

$I=$ Kovats retention index,
$n=$ the number of carbon atoms in the smaller alkane,
$N=$ the number of carbon atoms in the larger alkane,
$z=$ the difference of the number of carbon atoms in the smaller and larger alkane,
$t_{r}=$ the retention time.