Hello Dear,
If i understand you question you are analysing Acetaminophen(paracetamol) and you are using a different standard to try to quantify 3-acetamidophenol (metacetamol)...seems like they both are position isomers. I dont know the reason why but i would recommend using same standard to analyse it because the response factor could be different even at same molecular mass. But if its only a question of conversion the calculation would be:
Area sample/area std * concentration std/diluting factor of sample*100%
since they have same molecular mass.
Hope it help
Willy the GC
