Abstract: In this paper, an Atmospheric Pressure Ionization Mass Spectrometer (API-MS) ispresented' API-MS method is studied for the determination of residual CO2 in high purity nitrogen. The interference of coexistent water with the CO2 and the method for removing the water by P2O5 trap are described. The influences of gas pressure on the CO2,concentration are discussed. The method precision is better than 10%.