[clef]

Hi:)
I just need help with one small question regarding a gas chromatography prac i did:

How could you be certain a peak in the sample was your intended analyte ( in this case it was ethanol in a sample of wine) and not another substance with a similar retention time?

Thanks.

[FeLiXe]

you never know. but if you have a high resolution and the peak is at exactly at the same retention time then it's probably the same.

[chiralic]

@ FeLiXe and clef:  we have a technique called SPIKING....

This technique involves adding a known substance to the unknown. If this known substance is also present in the unknown, then the peak for that substance will be larger in the spiked sample than in the the unspiked sample. If the known is not a substance present in the unknown, then an extra peak will appear in the spiked sample compared to the unspiked sample.

@clef, in your case you would have to add ethanol to your wine sample

Also, you have several techniques pre and post column

Regards,

Chiralic

[clef]

Oh yeah thanks:) actually we added propanol as the internal standard. So we cant actually be sure that the 2nd peak is ethanol? I added ethanol to the standard solutions. 1ml ethanol to s1, 2ml of ethanol to s2 etc. So having the same retention time, we cannot be completely sure that it is the ethanol peak?

[chiralic]

Question: Do you have a gas chromatogram of your wine sample? Could you post it?

...actually we added propanol as the internal standard. So we cant actually be sure that the 2nd peak is ethanol?

You must do run a "sample": ethanol + propanol into your gas chromatograph....check the chromatogram...Then you must add AGAIN ethanol to your "sample", run this sample into your gas chromatograph again and then compare the both chromatograms, you'll see how increase the ethanol peak onto your 2nd chromatogram...this is the SPIKE Technique...

Then try to do this with your wine sample adding Ethanol (no add Propanol) to "identify" ethanol in your wine sample. If the peak high of ethanol increase, you can think that is Ethanol. Remember that you have another techniques and tools in Gas Chromatography (and in general for the all Separations Techniques) to IDENTIFY and to CONFIRM the presence of subtances into a sample.

I added ethanol to the standard solutions. 1ml ethanol to s1, 2ml of ethanol to s2 etc. So having the same retention time, we cannot be completely sure that it is the ethanol peak?

See the peak hight on yours chromatograms

[Ψ×Ψ]

See the peak hight on yours chromatograms

Welll...this is picky, but it's the integration rather than the peak height, isn't it?

[chiralic]

I think that in your first view of your chromatogram is more easy see "high" than "area", when you are using SPIKE TECHNIQUE (QUALITATIVE). When you are using Quantitative Technique (i.e. Internal standard) is most important integration(Area) than high.

Of couse if you have a integrator or computer device in you Lab, but if you have a recorder "sometimes not always" is better high that area...

[Ψ×Ψ]

Good point.  The GC/MSing I did over the summer was more quantitative-like.

[DrCMS]

If you really want to be sure a peak is the compound you think it is the two options i've used are GC-MS (even better is GC-MS-MS) or spiking the sample and runing it on two different conditions and or columns.  Under one set of conditions you may be unlucky and have 2 materials that co-elute but not under two different sets of conditions.

[Ψ×Ψ]

If you really want to be sure a peak is the compound you think it is the two options i've used are GC-MS (even better is GC-MS-MS) or spiking the sample and runing it on two different conditions and or columns.  Under one set of conditions you may be unlucky and have 2 materials that co-elute but not under two different sets of conditions.

GC-MS-MS pretty much requires a quadrupole ion trap, doesn't it?  I've seen things run on two completely different columns with EC detectors, and while the people doing the work seemed to have it down, I'm glad they were doing it and not me.  The two compounds I remember having the most trouble distinguishing between were anthracene and phenanthrene.  Same m/z, same elution time (I wonder if different conditions would change this?), similar fragmentation (though I don't remember exactly).