comparing data from non-polar and polar GC systems

[geruesgym]

Hello everyone!

Please go easy on me, if this question was posted before.

I've been analyzing samples of cuticular waxes of insects with a GC-MS with a non-polar DB-5 column. A collegue of mine identified the physiological active compounds among the samples - but he used a GC-MS with a polar DB-WAX column. We have a polar DB-WAX, too. But it is installed in our GC-FID.

How can I compare the data from the non-polar GC-MS and the polar GC-FID? The data I have available now are: alkane standards and samples measured with non-polar GC-MS, non-polar GC-FID, polar GC-FID.

I calculated the Kovats retention indices of the substances on the polar and non-polar system. But, of course, they are different. Is there a possibilty to compare the data?

Thank you for the help!
Daniel

[chromatographer1]

You have compared the samples against alkanes and have calculated kovats Indices on each type of GC phase.

That is the only comparison you can perform.

What else do you WISH to compare or to make any conclusions?

Do you want to ID a wax from the DB-5 data as the one that was seen on the Wax ?

Nope, not able to do that without MS data on the DB-5 column.

best wishes,

Rod

[Peter Apps]

IF the cuticular waxes are aliphatic hydrocarbons there might be something you can do, but the results and the conclusions that you draw from them will be tentative at best.

You have a non-polar chromatogram with the compounds identified by MS. Your colleague has a polar chromatogram with the compounds identified by MS. The polarity of the column cannot change the identity of the compounds, but it will change where each compound elutes. Using the compound identities you can map from one chromatogram to the other.

IF the waxes are aliphatic hydrocarbons, with limited double bonds and no odd molecular shapes I expect that you will find that the order of elution of the compounds on the two columns is quite similar. If you have pekas fo differnt sizes, and decent quantitative reproducibility between the systems then you can use peak size to resolve any changes in elution order.

You can use the same process to map from polar - FID to non-polar - MS, except that now you have to work only with elution order and peak size.

Be warned; your chances of ever getting this past a journal referee are almost zero. By far your best way forward is to install the polar column in the MS (or get you colleague to analyse more samples on polar - MS).

Question: why are you putting the non-polar MS results in the chain - why not go direct from polar - MS (where the identities and elution times of the actives are known) to polar - FID ? What information are you getting from the non-polar - MS ?

Peter

[geruesgym]

Thanks a lot for the answers!
To make it more clearly, I want to explain the backgriund of my question:

I did the analysis of the samples with the non-polar GC-MS during my master thesis and investigated changes in the composition of the mixture of cuticular waxes on the insect surface due to aging. There are quantitative differences of the mixture beween younger and older individuals. But to make a statement about wheather this is relevant for discrimination among the animals (e.g. males to be attracted only to to young females), I have to know which compounds are "physiological active". Now a collegue gave me these data (but not his GC runs), which he had collected with a polar GC system. Now I want to find his identified substances in my chromatograms. There are over 250 peaks and 20 "active" compounds. I calculated the Kovats retention indices, but I know that they can't be easily compared between polar and non-polar systems.

Do you think, repeating the analysis with a polar column in our GC-MS is the only way to align the different data?

best regards,
Daniel

[Peter Apps]

I think that you are making this too complicated - why not use the identifications from the non-polar MS to find the peaks on the non-polar chromatograms for the active compounds that were identified by your colleague. Forget about Kovats indices.

Peter