On-site chemical detection and monitoring of compounds related to homeland security applications, civil defence and forensics is difficult using conventional instrumentation. Target analytes include human chemical signatures (for detection of illegal immigration), drugs of abuse, explosives and chemical warfare agents (CWAs). A convenient solution is to complement existing techniques using portable membrane inlet mass spectrometry (MIMS). This thesis deals with the mass spectrometric investigation of characteristic chemical odour signatures emitted by human exhaled breath and skin as chemical signs of human presence in a confined space. It also presents detection results of threat and threat related chemical compounds. Numerical modelling of ion injection and confinement in a non-scanning linear ion trap (LIT) mass analyser for achieving sensitivity enhancement was carried out. A novel portable artificial sniffer based on linear ion trap (LIT) technology has been designed and developed. Initial performance results are described. Preliminary field trials have led to positive outcomes which are currently being commercially exploited.