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. In one embodiment about 90 % of the sample evacuated is diverted from the analyzer and recirculated into the air injector. In another embodiment a fan is provided to blow remnants of the sample cloud downstream of the test station. A hood may be provided in a shroud assembly at the test station to provide proper aerodynamics for the region for removal of those remnants.