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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!

Lia Kim
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    You may need different column, e.g. with more of aromatic character, better distinguishing existence or location of double bonds. – Poutnik Jan 26 '23 at 20:11
  • You can vary the GC column (capillary/packed; material of coating/packing; length of the column); the flow rate ($\ce{N2}$ vs. $\ce{H2}$ as carrier), the temperature of operation as fix (and the level as such can be altered), or as temperature gradient. Similar to liquid chromatography, programs may offer some guide of selection (e.g., EZGC chromatogram modeler). – Buttonwood Jan 27 '23 at 08:32

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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

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.

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).
Mathew Mahindaratne
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