GC/MS identifies 2 peaks of the same compound ?

[haiedc]

Hi all,

I have this problem for quite some time. In running essential oil in GC, I sometimes meet the situation of 2 peaks at different retention times but identified as one compound. The library qualities are rather high: about 95%.
Have anyone ever seen this and what is possible causes for this

Thanks

[Peter Apps]

Apart from two compounds having very similar spectra it is also possible, dependinding on how the sample is injected it might even be likely, that you have peak splitting in the GC. How much do the retention time of the two peaks differ ?

Peter

[WK]

Haiedc,
What are the compounds in question?
If you work with essential oils you would know that sesquiterpenes and monoterpenes sometimes have similar spectra and just looking at the fit is not always helpful.
Best way to run standards on your own instrument and then search.
WK

[haiedc]

Thank you both for the inputs. One of the pairs is CADINA-1,4 DIENE. The retention times are 24 and 27 min respectedly. Just now we don't have the standards

[Fat Jonny]

isomers? cis/trans?

[WK]

haiedc,
In my opinion I would say that 95% is not an indication of a good fit.
I always visually inspect the spectra. Try searching with reverse fit.
WK

[tangaloomaflyer]

Best way is to get hold of a pure standard if you can.

[Don_Hilton]

A great help in the world of essential oils is retention index. Take a look at www.flavornet.org. Retention indicies are specific to stationary phases. This site has retention indicies for up to four stationary phases. (If you are using an RTx-1, DB-1 or such, the column to look at is the OV-101 column).

You do not always have to compute the retention index. If you know enough about your sample, you can find a few peaks you recognize and use the retention index table to put the rest in order.

[thohry]

Could be some kinds of isomers : cis -trans or position isomers