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Topic: GC/MS to Identify Unknown  (Read 7443 times)

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Offline jadeite78

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GC/MS to Identify Unknown
« on: September 14, 2012, 11:48:04 PM »
I hope I get some replies.   I am trying to understand reading Chromatagrams.  I have an analysis,

Alkenes standards were injected first for testing (methane, ethene, ethane, propene, propane, butene, butane....)

Then 1 micoLiter of sample is injected.   How do I identify the compound on the graph that is between propene and butene with its time (min) 2.78 Area (μV*s) 23744532 Height 985159.91 (μV)

Offline discodermolide

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Re: GC/MS to Identify Unknown
« Reply #1 on: September 15, 2012, 01:20:02 AM »
I do not see the chromatography read out!
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Offline jadeite78

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Re: GC/MS to Identify Unknown
« Reply #2 on: September 15, 2012, 01:25:19 PM »
Here is the Chromatagraphy readout.   I should have better stated it.  First Alkane standards were injected, then Alkenes Standards.  I was speaking to someone and they said the compound is was closer to Butene. 1-Butene is around 3.99-4.0.   I need some help please.

Offline discodermolide

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Re: GC/MS to Identify Unknown
« Reply #3 on: September 15, 2012, 01:32:50 PM »
How did you inject 1 microliter of methane etc, which are gases? We're they dissolved in anything?
What were the GC conditions?
Is it butane?
« Last Edit: September 15, 2012, 01:50:17 PM by discodermolide »
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Offline jadeite78

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Re: GC/MS to Identify Unknown
« Reply #4 on: September 15, 2012, 01:50:06 PM »
All I know is a GC7 was used, the instrument checked for alkanes standards, then alkene standards, then my sample.  No alkanes in the readout, but there were alkenes.  The readout just states 1 μL of sample

Offline jadeite78

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Re: GC/MS to Identify Unknown
« Reply #5 on: September 15, 2012, 02:54:03 PM »
How did you inject 1 microliter of methane etc, which are gases? We're they dissolved in anything?
What were the GC conditions?
Is it butane?

No it was stated the compound structure is closer to Butene.

Offline jadeite78

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Re: GC/MS to Identify Unknown
« Reply #6 on: September 15, 2012, 02:55:50 PM »
Nevermind, it seems that no one knows how to read these graphs. I will figure it out sooner or later.

Offline discodermolide

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Re: GC/MS to Identify Unknown
« Reply #7 on: September 15, 2012, 05:00:25 PM »
Your title of the post states GC/MS, where is the MS part?
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Offline Babcock_Hall

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Re: GC/MS to Identify Unknown
« Reply #8 on: September 15, 2012, 05:49:19 PM »
How did you inject 1 microliter of methane etc, which are gases? We're they dissolved in anything?
What were the GC conditions?
Is it butane?

No it was stated the compound structure is closer to Butene.
Ordinarily one runs and reports the retention times of standards along with the unknowns.  Your retention time is less than that of 1-butene, but it seems to be greater than the retention time of propene.  Could it be an isomer of 1-butene?  Although I would not take it to be an absolute rule, GC retention times sometimes are correlated with boiling points.  I echo Discodermolide's question about mass spectrometry, and I think that without standard retention times or without mass spectrometry, one needs at least some sort of additional information.  I suggest that you draw out the isomer(s) of butene.
« Last Edit: September 15, 2012, 05:58:47 PM by Borek »

Offline jadeite78

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Re: GC/MS to Identify Unknown
« Reply #9 on: September 15, 2012, 08:40:51 PM »
How did you inject 1 microliter of methane etc, which are gases? We're they dissolved in anything?
What were the GC conditions?
Is it butane?

No it was stated the compound structure is closer to Butene.
Ordinarily one runs and reports the retention times of standards along with the unknowns.  Your retention time is less than that of 1-butene, but it seems to be greater than the retention time of propene.  Could it be an isomer of 1-butene?  Although I would not take it to be an absolute rule, GC retention times sometimes are correlated with boiling points.  I echo Discodermolide's question about mass spectrometry, and I think that without standard retention times or without mass spectrometry, one needs at least some sort of additional information.  I suggest that you draw out the isomer(s) of butene.


Well it is 64ppmv  I am taking it this was from MS.  That was the first thing I started doing was looking at boiling points of alkenes between propene and 1-butene.  But I will take your advice on drawing out isomers of butene.  Still frustrated.  But thanks though.

Offline jadeite78

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Re: GC/MS to Identify Unknown
« Reply #10 on: September 16, 2012, 04:41:13 PM »
why i have a negative mole snack??

Offline Babcock_Hall

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Re: GC/MS to Identify Unknown
« Reply #11 on: September 16, 2012, 06:57:17 PM »
Quote
Well it is 64ppmv  I am taking it this was from MS.  That was the first thing I started doing was looking at boiling points of alkenes between propene and 1-butene.  But I will take your advice on drawing out isomers of butene.  Still frustrated.  But thanks though.

I am not familiar with ppmv as an abbreviation.  What does it mean?  There are three isomers of 1-butene.  I can tell you from experience that isomers of heptene have different retention times by GC.

Offline discodermolide

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Re: GC/MS to Identify Unknown
« Reply #12 on: September 16, 2012, 11:58:36 PM »
I think it means Parts Per Million by Volume.

Although I do not see this anywhere on the GC print out. I think something is missing here.
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Offline DrCMS

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Re: GC/MS to Identify Unknown
« Reply #13 on: September 17, 2012, 04:53:22 AM »
why i have a negative mole snack??

I took one off you for:

1) posting a chromatography question without any chromatograph
2) listing your question as GC/MS and not posting any MS info
3) getting huffy when nodody gave you the answer you wanted

You are wasting people's time by not giving enough info and then wondering why nobody can give you the correct answer. 
From the info you have supplied so far there is NO answer anyone can give you that is anything more than a wild guess. 
You might know what you did during the experiment but you have not explained it clearly and have had to have the info required teased out rather than laying out a decent question with the correct info from the start.

Start by listing exactly what you did personally and what info you were supplied with. 

Offline Babcock_Hall

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Re: GC/MS to Identify Unknown
« Reply #14 on: September 17, 2012, 09:09:07 AM »
why i have a negative mole snack??
Don't worry about that; just try writing the structures of three isomers of 1-butene, and you will at least be closer to an answer than you were before.  I agree that some information seems to have been left out of the problem; therefore, if you have any more data for us to go on, then please supply it.

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