This may not be your best approach to determine peak purity

The response of a GC will ideally only be affected by the concentration of your analyte in the sample. However, in reality this is never the case. The response will be affected by alot of external parameters that will cause the signal to drift over time. A response factor is therefore not a good way to measure purity.

Thanks for your contribution

Thank you for the contribution

But how could the percentage composition be determined from the peak areas of GC-FID spectrum then?

In general,Rf is the ratio of the response value of the compound of interest and that of any standard compound whose identity and response value is established.

There are various methods to calculate response factor eg, (i) addition of internal standard,(ii) By external std. addition etc.