Atmospheric pressure chemical ionization mass spectrometry (APCI-MS) is being routinely used to quantify trace impurities in bulk gases used in the manufacture of semiconductor devices. APCI-MS has been successfully applied for the quantification of ppt (ng/Kg) levels of O-2, H2O, CO2 and CH4 in Ar, N-2 and He and trace N-2 in He. However it has not been successfully used to quantify ppt levels of N-2 in Ar due to unfavorable rate constant for the charge transfer reaction between Ar+ and N-2. We report here the detection of ppt levels of N-2 in Ar by adding a few percent of purified H-2 into the ionization source of the mass spectrometer. The addition of H-2 results in a plasma dominated by protonated ions and the detection of trace N-2 is facilitated by favorable proton transfer reactions to produce N2H+. Our experimental results indicate statistical detection limits below 50 ppt.