Abstract: A method of detecting contraband substances in freight cargo containers in which the container is agitated to disturb particulates therein, and air containing such particulates is then sampled and the particulates collected. The collected particulates include naturally occurring particulates which have absorbed vapors of the contraband substance during the entire time that the container has been closed, and also include particulates of the contraband substance itself. The collected particulates are heated to drive off vapors indicative of the contraband substance and the vapors are analyzed in a mass analyzer.