Separating alkane/alkene isomers on the GC but the isomers have the same retention time

I'm trying to differentiate between n-octane, 3-octene, and 4-octene from a mixed sample using gas chromatography, but n-octane and 3-octene seem to have the same signal and 4-octene is shouldering the n/3 peak. Does anyone have suggestions for a way to separate the peaks? I've currently been trying to change the temperature method but the ones I'm trying seem to not be working. Thanks!




For our organic chemistry lab program we have developed GC program for one of the separation labs and we use alkane homologues as the analyte mixture to be separated. The chromatogram give a baseline separation on shorter (3-m column) Carbowax-20M packed column. Thus, for curiosity, I checked for separation of alkene isomers on similar column. Sure enough, I found one separating $\ce{n-C16H32}$ alkene isomers:


[![GC of n-Hexadecene](https://i.stack.imgur.com/R4Ox5.jpg)](https://i.stack.imgur.com/R4Ox5.jpg)


The column is $\pu{300 m}$ Garbowax-2OM capillary column operating at $\pu{96 ^\circ C}$ with $\pu{0.15 MPa}$ $\ce{N2}$ mobile phase (Ref.1). As indicated in the diagram, the method has even separated most of $cis/trans$ isomers at same position.




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**References:**


1. Ladislav Soják, Ján Krupčík, and Jaroslav Janák, "Gas chromatography of all $\ce{C15}$-$\ce{C18}$ linear alkenes on capillary columns with very high resolution power," *Journal of Chromatography A* **1980**, *195(1)*, 43-64 (ODI: <https://doi.org/10.1016/S0021-9673(00)81542-3>).


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