Given the retention time obtained from the GC-MS analysis of essential oils from the hydrodistillation of leaf of plant?
The original Kovats paper explains this in detail (DOI: 10.1002/hlca.19580410703), but briefly, you should inject a standard solution of n-alkanes with carbon numbers and retention times immediately above and below those of your target compounds in your samples (typicallyC8-C20 Fluka #04070, or C10-C25 Restek #31214). For each compound (peak) in your sample, find the two n-alkanes that immediately precede and follow its retention time, and apply the following equation.
I= 100*[ n + ( Tu-Tn)/(TN-Tn)]
where:
I = Kovats retention index value of your unknown compound (peak)
n = the number of carbons in the alkane preceding your compound
N= the number of carbons in the alkane following your compound
Tu = the retention time of your unknown compound
Tn = the retention time of the preceding alkane
TN = the retention time of the following alkane
If you are doing isothermal chromatography, there is a more precise equation you can follow.
In theory, any set of regularly occurring, stable compounds of systematically increasing mass and constant function moieties can be used to establish a system of retention indices. However, the Kovats index is the most widely used and studied.
Indices such as these are only comparable on similar column phases operated under comparable temperature profiles, and without on-column decomposition or column overloading. Very stable temperature profiles are key to useful RI inter-comparison for compound identification.
Briefly goal, you need to inject a standard solution of n-alkanes with carbon numbers and retention times time immediately Above and below your target Those of compounds in your samples. For Each compound peak in your sample, find the two n-alkanes and time immediately PRECEDING Following icts retention time, and apply The Following equation.
I = 100 * [n + (Tu-Tn) / (TN-Tn)]
gold:
I = Kovats retention index value of your unknown compound (pic)
n = the number of Carbon Atoms in the alkane your PRECEDING compound
N = the number of carbons in the compound after-your alkane
T = the retention time of your compound unknown
Tn = the alkane PRECEDING the retention time
TN = The Following alkane retention time
In theory, any set of Regularly Occurring stable increasingly Systematically weight compounds and moieties constant function can be used to ESTABLISH a retention index system. HOWEVER, the Kovats index is The Most Widely Studied and used.
Such indexes are only comparable phases of similar columns made in comparable temperature profiles, and without decomposition on column or column overloading. Very steady temperature profiles are key Useful RI intercomparison for identification of compounds.
Retention index of the constituents of essential oil can not be determine directly from GC/MS data. One is required to run the GC of the oil with n-alkane and note the retention times of alkane that was detected before and after an essential oil constituent. Then estimate the retention index as follow: Retention time of oil constituent -retention time of alkane detected before oil constituent divided by difference between the retention times of alkane that were detected before and after the detection of the oil constituent plus the number of carbon in alkane that was detected b4 the oil constituent multiply by 100.
@Luke A Ackerman what is n-alkane standard mean?is it special standard?or we can use n-hexane as standard?thank you
Am too is facing the same problem...can anybody explain with proper example s...
mr luke explanation is very excellent i left a persian article about calculation of kovates indix with DP-5 colum
Adia Putra Wirman . I am sorry I just saw your question from some years ago. Yes there is a standard mixture of n-Alkanes (not just one) from which most Kovats retention index's are calculated. Typically C8-C20 at ~40ug/mL.
https://www.sigmaaldrich.com/US/en/product/sial/04070
Luke K Ackerman what about the fruit cuticle wax standard use for GC MS? Do they need to use it separately or is a standard mixture?
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