Dear Benmeddour Tarek it is very difficult to determine KI or RI without the retention time of n alkane you need the GC analysis for the identification of the oil samples.  

Mass spectra may be compared with those reported in available libraries as NIST. This may lead some useful informations.

In many cases these libraries are integrated into the software of GC-MS instrument.

However for a correct identifiction of the components also a GC-FID determination with a series of n-alkanes as standard compounds is needed.

Latest MS comes along with KI values of some constituents are already in the NIST libraries.

The best procedure is to identify alongside the n-alkanes series and then use the KI calculation formula.

You cannot really DETERMINE KI values without the retention times of n alkanes since the calculation requires these retention times.  You really want to use these KI's as an extra piece of data to support your mass spectra so you need to actually measure them.  The only alternative might be that if you are certain of the identification of a number of your terpenes you could  plot their KI's versus retention time and use that graph to determine other values.  The trouble is the MS of terpenes are not always very characteristic and that is why you must calculate the KI's independently.

Remember that RI and KI values are not absolute but comparative to reference/standard compounds. They are also dependent on the type of column used. You therefore need standards. GC-MS will suggest hits from the internal libraries (not exhaustive) which will need to be confirmed by comparison with authentic samples, RI or KI

You did not mention the name of the essential oil or the name of the plant from which you have isolated the essential oil. If your purpose is to simply identify the components of your essential oil you can compare your oil with reported composition for identifying some of the oil components. If it is a new essential oil not investigated by any researcher previously, you need to follow the advise of the above researchers. 

You can't determine the KI values without the retention times of the alkanes because  the calculation of KIs requires them.  To use the KI's you will need to  measure the retention times.

In my opinion, you should consider, if practicable, to run n-alcanes (usually C8-C20 is a good choice) at the same conditions you analysed the essential oil (identical method, column, etc) and use results to calculate the indices (KI for isothermal and Non-isothermal Kovats retention indices (from temperature-programming, using definition of Van den Dool and Kratz: http://webbook.nist.gov/chemistry/gc-ri/). 

No problem.

Use DB-5 column and the RI table from R.Adams "Identification of essential oil components by gas chromatography/mass spectrometry" 2007. Assign several RI values to identified terpenes and use it as repers in AMDIS or UniChrom.

Dear Tarek

I am attaching an old but highly popular publication on KI or RRI.

Good luck.

The only alternative might be that if you are sure of the identification of a number of your terpenes you can draw curve and use this chart to determine other values of their KI versus retention time .

Whatever the case you need essential oil standards for co-injection and peak enhancement for unambiguous identification of the components