A method for securing qualitative and quantitative information of a high molecular weight additive in a polymer resin sample is disclosed herein. In some embodiments, the method includes separating a fraction of a polymer resin sample using size exclusion chromatography (SEC), wherein the fraction corresponding to a high molecular weight additive, pyrolyzing the fraction in a pyrolysis-gas chromatography/mass spectrometer (Py-GC/MS) to obtain a mass spectrum of the pyrolyzed fraction; identifying a structure of the high molecular weight additive by comparing m/z values for fragment peaks in the mass spectrum to m/z values for fragment peaks in a mass spectrum of a standard, and determining the amount of the high molecular weight additive in the polymer resin sample, relative to the total weight of the polymer resin sample by comparing a sum of areas of the fragment peaks to a calibration line of the standard.