Question

What is a Kováts retention index in gas chromatography, and how do I calculate it?

Answer 1

The Kováts retention index (or Kováts index) of a compound is its retention time normalized to the retention times of adjacently eluting n-alkanes.

Explanation:

It depends on the fact that `logt ∝ n`, where `t` is the retention time and `n` is the number of carbon atoms in the alkane.

For an isothermal chrtomatogram, you use the following equation to calculate the Kováts index, `I`:

`I = 100 × [n + (logt_x– logt_n)/(logt_(n+1) –logt_n)]`, where

`n` is the number of carbon atoms in the n-alkane, and `t` is the retention time.

First, you run a chromatogram of a standard alkane mixture in the range of interest, say from `"C-8"` to `"C-16"`.

You might get something like the chromatogram below.

Then you do a co-injection of your sample with the standard alkanes.

Assume that the retention times were: sample = 3.12 min; `"C-9"` = 2.71 min;
`"C-10"` = 3.89 min. Then the Kováts index for your sample is

`I = 100 × [n + (logt_x– logt_n)/(logt_(n+1) –logt_n)] = 100 × [9 + (log3.12 – log2.71)/(log3.89 – log2.71)] = 939`

Even many years later and in a different laboratory, you can reproduce this retention index even though your old retention times no longer have any meaning..