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To accurately measure Cl2mixing ratio using stack emission monitors and leak monitoring sensors in a workplace, it is essential calibrating monitors and sensors periodically using a Cl2standard gas mixture. Because there is no reliable analytical technique for Cl2, an international standard for high-pressure Cl2gas mixtures has not been established. A novel approach for the quantification of Cl2was developed using quadrupole mass spectrometry (QMS) with an expanded uncertainty of 0.7 µmol mol-1. A 100 µmol mol-1chlorine (Cl2)/N2gas mixture was produced through a two-step procedure involving dilution of Cl2with N2in high-pressure aluminum cylinders. To check the consistency between 100 µmol mol-1Cl2gas mixtures in different cylinders, Cl2gas mixtures were quantified using the QMS based approach. It was found that four cylinders of 100 µmol mol-1Cl2gas mixtures prepared in 2016 agreed within 0.7 µmol mol-1. The long-term stability of 100 µmol mol-1Cl2gas mixtures was assessed through changes in the Cl2mixing ratio over a one-year. It was found that the 100 µmol mol-1Cl2gas mixture was stable within ±0.7 µmol mol-1over one year. Finally, 100 µmol mol-1Cl2/N2gas mixture was successfully developed in a high pressure cylinder with an expanded uncertainty of 2.0 µmol mol-1(k= 2; 95% confidence level).