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Author Topic: Gas chromatography to work out percentage composition  (Read 4037 times)

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dexxt

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Gas chromatography to work out percentage composition
« on: May 03, 2012, 02:19:24 AM »

Hi long-time lurker in need of help:

Basically I did a series of coupling reactions in the lab each with slightly different conditions and want to be able to compare how much product I made in each case. 

As I was working on a small scale, instead of trying to isolate product, my supervisor told me to just use the GC to see if there was any present.  There was and now I want to be able to compare how much product I made in each case.  I don't need to know the exact amounts, just have an idea of percentage yields so that I can say which reaction was better.  I know I shouldn't just compare the peak areas directly (or should I?) so how do I do this?  ???

Each spectra have 5 peaks: 1 right at the start for the reaction solvent, 1 for the product, peaks for starting materials A and B and 1 peak for the catalyst. 

The easiest way (I think) would to have just done a calibration curve however supervisor said this wasn't necessary so I ran a spectra of a mixture of known concentration of starting material A and the product.  However as I didn't put an internal standard in my mixtures before analysis, I'm unsure of how to use that.

So how and can I compare how much product was made in each case?

Any help much appreciate :)
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JGK

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Re: Gas chromatography to work out percentage composition
« Reply #1 on: May 03, 2012, 09:04:08 AM »

Without calibration you can't really do an accurate quantfication but you can do a measurement based on the relative percentage area of the peaks. (this assumes that the unit response odf all the materials is the same)

Ignore peak 1 the solvent peak
Ignore the catalyst peak this should be roughly the same in all the chromatography any way.

At t= zero you should have 100% of the starting material.

As Products A and b begin to form you can measure the Area% of the peak of Interest (POI, this could be the starting material, Product A or B) as follows:

POI Area% = Area (POI)x 100/(Area (starting material)+Area(Product A)+Area (Product B))
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dexxt

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Re: Gas chromatography to work out percentage composition
« Reply #2 on: May 04, 2012, 07:52:03 AM »

thanks for taking the time to reply JGK

Think I may have misled you in that I have 2 starting materials to make 1 product, not the other way around.  My reaction is basically:

A + B  :rarrow: product

But I guess your equation still applies:

POI Area% = Area (POI)x 100/(Area (starting material A)+Area(starting material B)+Area (Product))

Do I not need to include some adjustment for response factor?  I'd assumed that was why I was told to do a spectra of known amounts of product and starting material but without an internal reference in my samples was confused about how to include this.
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JGK

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Re: Gas chromatography to work out percentage composition
« Reply #3 on: May 04, 2012, 09:30:15 AM »

thanks for taking the time to reply JGK

Do I not need to include some adjustment for response factor? 

I'd assumed that was why I was told to do a spectra of known amounts of product and starting material but without an internal reference in my samples was confused about how to include this.

If you inject a known amount of any of the POIs you can calculate its unit response by dividing peak area by amount. Then for samples containing unknown amounts you can calculate the amount by dividing the area by the unit response. This assumes a linear relationship between area and concentration
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