In order to protect life and the environment, the emission of nitric oxides must be reduced and monitored. To allow the use of cost-effective analysis methods, e.g. infrared technology for monitoring the NO2 content in the emitted gas, the sample is catalytically converted to NO. This is achieved by using electrically heated small reactors filled with various catalyst materials for different applications. Energy-efficient operation of these NOx converters requires the operating temperature to be as low as possible. This also will prevent CO production. “Long life” reactor cartridges are available to ensure extended operation.