Method and arrangement for detecting harmful substances

A method for detecting gaseous harmful substances in a sample gas flow utilizing a measuring system including a metering unit for metering a reaction gas and a combination of an ion-mobility spectrometer and a plurality of detectors which detect those of said harmful substances not detectable by said ion-mobility spectrometer, the method comprising the steps of:

• drawing said sample gas flow into said measuring system;
• with a pump, admixing a reference gas to said sample gas flow before said sample gas flow reaches said detectors or, alternatively, only before said sample gas flow reaches said ion-mobility spectrometer so as to cause said sample gas flow to be mixed with said reference gas in order to always maintain the concentration of said harmful substances supplied to said detectors below a predefined concentration;
• during the measurement, supplying said reaction gas via said metering unit to said ion-mobility spectrometer when complex ion-mobility spectrometer spectra are present in order to thereby increase the selectivity of said ion-mobility spectrometer;
• obtaining measurement signals from said ion-mobility spectrometer and said detectors as to a gas-quantity ratio between said sample gas and said reference gas;
• adapting said measurement signals to the original concentration of said harmful substances in said sample gas;
• thereafter, comparing said measurement signals to previously defined and stored measured values via a simple comparison of the signal heights or a pattern recognition wherein all detector signals are compared to previously stored signals at the same time;
• based on this comparison, issuing an alarm;
• and,
• rinsing said measuring system by supplying said reference gas thereto after the measuring phase and supplying an additional cleaning gas to cleanse said measuring system when harmful substances are detected which can not be rinsed.