August 10, 2020, 06:04:08 PM
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Topic: GC separation of isomers  (Read 204 times)

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Offline Cantacoxinha

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GC separation of isomers
« on: July 02, 2020, 04:49:35 AM »
I need an opinion on a separation case. A mixture of (E)- and (Z)- isomers of the following compound is theoretically injected on an RXi 5ms (30m, 0.25mm, 0.25┬Ám) GC column. Will these two isomers separate and form two distinct peaks on that type of stationary phase or coelute as a single peak?

Offline DrCMS

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Re: GC separation of isomers
« Reply #1 on: July 02, 2020, 06:37:00 AM »
What do you expect and why? 

For that type of GC column what do analyates typically elute in order of? 
For that property how different are these two isomers?

Offline tlahren

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Re: GC separation of isomers
« Reply #2 on: July 27, 2020, 06:01:20 PM »
If this is for a test question I would imagine they are looking for a "NO" answer as standard non-polar GC columns typically do not separate isomers "very well."  That being said, I think that E/Z isomers of this chemical "may" separate on a 30 m 5% phenyl column.  This would be based mostly on difference in BP and stearics which would be limited as one of the largest parts of the molecule (hexanol on the sulfide) is free to rotate on that -S- bond.  As shown, the E isomer seems it would have slightly larger molecular size than a Z isomer.  This would give it a slightly higher BP and MORE interaction on the stationary phase.  These features would lend to a later elution time than the E.  It is almost impossible to say if they would completely separate though.  The exposed hydroxyl group would make both peaks tail (asymmetrical back end) quite a bit and the peaks would not likely resolve to < 10% height.  This is just my guess.

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