In situ measurements of Krypton in Xenon gas with a quadrupole mass   spectrometer following a cold-trap at a temporarily reduced pumping speed
  A new method for measuring trace amounts of krypton in xenon using a cold trap with a residual gas analyzer has been developed, which achieves an increased sensitivity by temporarily reducing the pumping speed while expending a minimal amount of xenon. By partially closing a custom built butterfly valve between the measurement chamber and the turbomolecular pump, a sensitivity of 40 ppt has been reached. This method has been tested on an ultra-pure gas sample from Air Liquide with an unknown intrinsic krypton concentration, yielding a krypton concentration of $330 \pm 200$ ppt. 
Ethan Brown, Stephan Rosendahl, Christian Huhmann, Christian   Weinheimer, Hans Kettling
physics.ins-det hep-ex