Gaseous impurities in nitrogen ambient in the chamber of an atmospheric pressure rapid thermal processor were quantitatively measured. We employed atmospheric ionisation mass spectrometry (APIMS) for this purpose. APIMS is the most sensitive technique to detect trace impurities in a gas at atmospheric pressure. A wide dynamic range (0.1 ppb - >10 ppm) measurement was successfully performed, which allowed real-time monitoring of impurities during rapid thermal annealing. This work reports fundamental behaviour of the ambient impurities originating from different sources. The sources discussed in this paper are fourfold: source gas, system background, air (wafer loading), and wafer itself. The contribution of the source gas was found to be negligible, whereas the air and the wafer were found to be most crucial. Ambient management requires a better understanding of the independent contribution of each source to processing. It is shown how in-situ measurements help to define process recipe for different types of wafers.