For controlling and monitoring of fusion deuterium plasmas and for helium leak-testing of vacuum vessels as well as for experimental investigations of possible effects of selective gas pumping during cryopumping, it is necessary to detect helium in a deuterium atmosphere, which stands as a model gas for deuterium/tritium mixtures. To cope with that, a novel quadrupole gas mass spectrometer (BALZERS GAM 400) has been procured and tested, which allows for base-line separation of helium (mass 4.0026 amu) and deuterium (4.0282 amu) in a high-resolution mode and also offers the conventional operation mode (mass range up to 128 amu) by two different ion-source parameter sets. Thus, it is possible to analyse multi-component mixtures quantitatively with a high He/D 2 resolution and a sensitivity for other gases in the 10 ppm-range. The mass spectrometer is placed outside the test vessel and equipped with two different gas inlet systems from the test vessel to cover the whole process pressure range between 10 -4 mbar and atmospheric pressure. The performance characteristics of the gas analysing module are presented. Experience gained by using the analyser to monitor concentration shifts due to the selectivity in cryopumping of gas mixtures is also reported.