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GC elution order

  1. Oct 18, 2005 #1

    ShawnD

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    I ran a series of alcohols; 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol; through a GC with a polar column. For this particular column:
    -Polar molecules are held back more (true with polar columns)
    -Higher boiling molecules are held back more (true with all GCs)

    The problem is that these rules go against each other. The elution order based on polarity would be octanol, heptanol, hexanol, then pentanol because smaller molecules are more polar (ethanol dissolves in water, pentanol does not). The elution order based on boiling points would be pentanol, hexanol, heptanol, octanol because bigger molecules have higher boiling points.

    The lowest boiler of the bunch is pentanol, and that was at something like ~170C iirc. The injector and detector were 250C, the column itself was 75C. I'm not sure if this low temperature means the boiling point has more or less importance. When I ran decane, undecane, and dodecane through a nonpolar column, there was a very large difference in elution times and they were based solely on boiling point since each of those alkanes have basically the same nonpolarity.
     
    Last edited: Oct 18, 2005
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  3. Oct 18, 2005 #2

    GCT

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    from the short experience that I had with chromatography, the molecular weight was highly correlated with the elution order at times the polarity made a difference, with Si as the stationary phase.
     
  4. Oct 18, 2005 #3

    movies

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    GC columns rarely use Si solid phases. It's usually some sort of wax column.

    It's hard to say which effect will take precedence though. I guess in my experience it's usually more of a BP effect, but I don't have a ton of experience with GC and very polar compounds. Molecules with similar MW and somewhat different polar groups often aren't that different on GC, but two non-polar molecules with a moderate difference in MW sometimes have massively different GC behavior.

    The handful of very polar molecules I shot onto a GC never came off, which wasn't entirely unexpected though.
     
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