The invention is a method and system of screening the content of an enclosure, such as a cargo container, for the presence of one or more target substances, such as explosives or drugs, comprising drawing air from the enclosure and passing the drawn air across at least one sampling card having a coating configured to absorb/adsorb the one or more target substances and thereafter analyzing the sampling card to determine if the coating has absorbed/adsorbed one or more target substances. The system embodiment includes a vacuum source, a conduit coupled to the vacuum source and a sampling card holder disposed along the conduit. The sampling card holder removably holds at least one sampling card having a coating thereon configured to absorb/adsorb the one or more target substances, so that air drawn into the conduct from the enclosure passes across the at least one sampling card.