I calibrate my GCs externally using verified Fluorochem, PCR, Apollo gas standards. The column I use is a Chrompack PLOT Al2O3/NaSO4. In the past I have diluted these standards in air (using a gas burette) prior to GC injection. I was asked to amend my calibration procedures so that I calibrated in the matrix I am tring to analyse (i.e making up calibrants in purified product). Can anyone explain why the response factors should be so different ie ca. 40% higher?

I know that this might be stupid, but you did check your diluent matrix to ensure that it does not contain peaks of interest? Also checked all delivery systems to ensure no contaminations?