Gaseous impurities in the chamber of a SHS2800ε rapid thermal processor were quantitatively measured by using atmospheric pressure ionisation mass spectrometry (APIMS). APIMS is a very sensitive technique to detect trace impurities in a bulk (1 atm) gas. A wide dynamic range (0.1 ppb - 10 ppm) measurement was successfully performed, which allowed in-situ monitoring of impurities during RTP. This work reports the fundamental behaviour of ambient impurities originating from different sources. The sources discussed in this paper are threefold: system background, wafer loading, and the wafer itself. Ambient management requires a better understanding of the independent contribution of each source on processing.