Following 1.5 years of shutdown and then a lab move, our 5975 is maintaining vacuum with the ion gauge reading 6e-6 torr. The GC will maintain either 1 bar or 30 mbar of He flowing at 10-200 ml/min, the conditions don't matter. With the instrument in standby (MS off) and seemingly at random, the MSD vents and cools down, but the GC still maintains its pressure and flow conditions. The m/z ratios for 28, 32, and 18 relative to 69 are 8%, 1.4%, and 0.4%, and 28/69 increases when the MS is on because of contamination of the source housing (visually observable), however the venting issue happens with the MS off. There is no clear indication of an air leak. What else might cause the MSD to vent besides air? Might this be a failing electrical component.
