A system and method are provided for minimizing the effects of background signals in masking signals indicating the presence of substances to be detected such as contaminants in materials moving rapidly along a conveyor. The contaminants detected may include nitrogen containing compounds and hydrocarbons. The system and method of the present invention minimizes the number of falsely positive indications of the presence of such substances due to background signals and changes in background signals. The substances detected are divided into first and second sample portions and the respective portions are heated. The first heated portion is reacted with ozone to generate radiation by chemiluminescence having characteristic wavelengths related to substances in the first portion. The second portion heated is also reacted with ozone to generate radiation by chemiluminescence having characteristic wavelengths related to substances in the second portion. The radiation of the respective first and second portions is selectively detected. The heating and detecting steps are performed in a manner so as to yield a higher level of detected radiation from one of the portions of the sample then the other for at least some of the selected compounds being detected. Electrical signals from the respective first and second portions are generated and compared in order to determine the presence or absence of selected compounds in the sample. Appropriate reject signals for a bottle sorting system are generated accordingly.