Chromatography Forum

How close the kovat index is enough to guarantee these two compounds can be co-eluted? like plus minus 5.

It can be tough to tell. I've seen Kovats indices for the same compound and same column differ by as much as 20 units in the literature. I use kovats mainly to narrow down what an unknown could be. For example with flavors Anethole and Estragole have the same EI spectra. Eugenol and isoeugenol, many other isomers as well (though not all) are not distinguishable by EI. As a result I use the approximate kovats (I have compounds that are known elute before that I can use as markers).

this site is very useful.
http://www.flavornet.org/f_kovats.html

Relative retention times (which are what Kovats indices express) depend on stationary phase, gas flow rate, column manufacturer (especially for polar phases), temperature programme, sample composition, column deterioration etc. The only way to be sure that your peak is eluting at the same time as another compound is to run them both under the same conditions (including solvent and injection volume), and the best way of all is to spike the compound into the sample and see if you get symmetrical co-elution.

Bear in mind that there are way more chemicals around than elution times from a column, so co-elution does not, on its own, confirm identity. Failure to co-elute does mean that you have a different substance.

Peter