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On 9 August 2000 a fire started at a facility that manufactures pool chemicals in Guelph, Ontario. A mobile trace atmospheric gas analyzer (TAGA) unit was summoned to provide on-site air monitoring operated by the Ontario Ministry of the Environment (OMOE). The responsibility of the TAGA unit was to monitor in real time the airborne contaminants released through the combustion of pool chemicals. This was accomplished by using an atmospheric pressure chemical ionization (APCI) source with the newest TAGA (model IIe): a triple quadrupole mass spectrometer which allows for the direct sampling and real time analysis of air for a wide range of toxics at low parts-per-billion (ppb) levels. The ionization mechanism under negative APCI conditions is dominated by charge transfer reactions, yielding parent ions which are selected in Q1, dissociated in Q2 and the resultant daughter ions are identified in Q3. By monitoring specific parent/daughter (P/D) ion pairs, the TAGA IIe was able to simultaneously measure, in real time, levels of hydrogen chloride (HCl) and chlorine (Cl2) present in the air. The response of the TAGA IIe was characterized by multi-point calibration curves which were linear up to 250 microg/m3 for HCl and up to 600 microg/m3 for Cl2. The average detection limit (DL) for this application was 0.50 microg/m3 for both HCl and Cl2. On-site measurements of HCl and Cl2 were made at several locations upwind and downwind of the fire site over a period of 3 days. The data collected by the TAGA unit was used by the local officials for a real time assessment of the airborne levels of HCl and Cl2.