Abstract: A method and apparatus are disclosed for detecting the presence of very fine leaks through the walls of an enclosure which include a sniffer nozzle (1) open to the atmosphere, a vessel (5) having molecular sieves (4) therein for adsorbing substantially all gaseous products which have entered the sniffer nozzle (1) with the exception of the probe gas which has been pressurized within the enclosure being tested, a pump system (8, 9) for creating vacuum conditions within the system, a mass spectrometer (12) for detecting the presence of the probe gas, and a capillary tube (2) interconnecting the nozzle sniffer (1) and the molecular sieve assembly whereby the probe gas draw rate, and sensitivity of the apparatus of the present invention, are significantly increased.