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In gas chromatography, the number of peaks represents the number of compounds in an unknown sample, as retention times for each component differ.

But what would happen if two components have the same exact retention time? Would you see two separate peaks or one?

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    $\begingroup$ Please avoid clickbait terms in titles (ie "urgent") $\endgroup$ – Buck Thorn Feb 28 at 16:08
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    $\begingroup$ Why would you get two? $\endgroup$ – Buck Thorn Feb 28 at 16:09
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] Complete overlap (= identical retention) is a regular feature of experimental chromatography with very different compounds and complex mixtures. It just happens when the selectivity happens to be equal to 1.00. You can have one, two, or three different compounds hidden under a single peak and this usually happens in early eluting peaks in all modes of chromatography of a complex mixture.

Peak overlap is undesirable, but it happens all the time. All you need to do is to change the method.

People have spent a portion of their life in something called as peak deconvolution to find out what is hidden in a single peak. Note how the area adds up.

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Have a look at this one from reference (1) [my highlight]:

In the simple case, the use of retention data for identification purposes involves merely a comparison between the retention time of an unknown and that of a series of authentic compounds. Since the essential ability of chromatography is to separate substances, a negative conclusion - these two compounds can be separated, thus they are different - can be reached with certainty. A positive identification, on the other hand cannot; there are just too many compounds which may have the same retention time. The odds can be improved very simply, by just measuring the column efficiency for the unknown and for the mixture containing both the unknown and the suspected compound in the same amounts. If the measured efficiency is the same and if its precision is about 108, the compounds could not be resolved with a resolution unity, by a column about 10 times more efficient than the column used. We must emphasize again that a positive result for this test is not a proof that the two compounds are identical.

I firmly recommend at least two reliable methods of identification, ideally more, not just gas chromatography.

Sources

  1. Guiochon, G. and Guillemin, C.L. (1988) Chapter 11. Qualitative analysis by gas chromatography: The use of retention data., in Quantitative gas chromatography, J. Chromatography Library 22:481-529. DOI: 10.1016/S0301-4770(08)70083-6
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Strictly same retention time means exact same signal in GC (i.e."apparently" same compound). This means you'd have one peak for the mixture as if it was a single compound. On an unknown sample, you might miss the fact that there are two compounds coeluting.

To avoid the issue, you would always inject the sample on (at least) two columns with different polarities. If your GC is set up properly, you can do this in parallel without additional effort, it is definitely good practice. I have been trained to use gas chromatography that way.

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  • $\begingroup$ The signals add up. If compound A and B peaks merge into a single peak, their areas should sum up. It is a good idea to test two orthogonal columns. $\endgroup$ – M. Farooq Mar 1 at 23:01
  • $\begingroup$ Or you could use GC-MS to determine peak purity. $\endgroup$ – theorist Mar 2 at 1:23

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