disclosed is a method and apparatus for sampling and determining the presence of certain substances , such as residues of contaminants in containers . the method includes steps of : injecting compressed air into said containers in order to displace at least a portion of the contents thereof ; evacuating a sample of the container contents so displaced by applying suction thereto ; and analyzing the sample evacuated to determine the presence or absence of the certain residues therein . the compressed air is injected through a nozzle into an opening in the containers to displace a portion of the container contents and form a sample cloud outside of the container . the sample cloud is then at least partially evacuated by suction and the sample is analyzed for the presence of contaminants such as nitrogen containing compounds or hydrocarbons . the sample cloud may be split into first and second portions . contaminants in the first portion are detected by a chemiluminescence detector and the second portion by a pulsed fluorescence detector .