This method is for determining trace acetonitrile and propionitrile in high purity polymer grade ethylene and propylene gas or liquid using a gas chromatograph equipped with a mass spectrometer run in selected ion monitoring mode. The specific instrument is preconfigured and is capable of determining typical impurities in polymer grade ethylene or propylene, however this method only outlines the procedure for determining acetonitrile and propionitrile. The lower limit of quantitation for acetonitrile and propionitrile are 0.02 mol- (or mass-) ppm.