Quantitative analysis via GC

Discussions about GC and other "gas phase" separation techniques.

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I am analyzing a reaction mixture by GC and I am facing trouble quantifying the desired product.

The technical department has gone through the process of calibration curve so, I have what They call a response factor (KF=0,63) between the standard and the product of interest. They determined this factor with a mass of standard = 18,90 mg and a mass of clean product = 19,3 mg.

After performing my reaction, I prepare a sample for GC analysis. To do that, I add in a vial a random weighted amount (mg) of the reaction mixture, the standard and the solvent. Then I submit the sample for GC.

I received a spectrum where there are different peaks. As I know the retention time of the product and the standard, it is easy to identify them. I use the following formula to calculate the amount of product I am having from the reaction: m(product)= Area(product)/Area (standard) * KF * m (standard)

Question: Calulating the mass following the above steps never give me the right amount of product I have (I have perform a purification of the reaction mixture to know how much I have and see if it was accurate). Is there any steps missing in my calculation?

Question: Also, as I have prepared a sample for GC (dilution of the reaction mixture), how does it represent the reaction mixture? I never used the dilution I have made for the calculation of the mass of the product. If I have to, how should I do that?

Thank you in advance for your help.
It will depend if your analyte and the standard respond to the detector in a linear fashion. If not linear, then the only accurate result will be near that of the calibration point. If the instrument is calibrated on a curve with multiple points then you have a more broad range at which you will have accurate results.
The past is there to guide us into the future, not to dwell in.
For the calibration to be valid the standards and sample must have been treated in the same way in terms of solvents, volumes, temperatures or whatever. The technical dept should have constructed the curve by spiking mixtures similar to what you are analysing. If they just dissolved standard and product in clean solvent, that is the likely cause of the discrepancy.

Peter Apps
3 posts Page 1 of 1

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