GC quantitative analysis

Hello, ​ 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 a ratio between the standard and the product of interest (0,63). ​ After performing my reaction, I prepare a sample for GC analysis. To do that, I add in a vial a known 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 product of amount I am having from this reaction: Area(product)/Area (standard) * ratio * m (standard) ​ My question is 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? ​ 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? ​ ​
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