Detection of krypton in xenon for dark matter applications
  We extend our technique for observing very small concentrations of impurities in xenon gas to the problem of krypton detection. We use a conventional mass spectrometer to identify the krypton content of the xenon, but we improve the sensitivity of the device by about five orders of magnitude with a liquid nitrogen cold trap. We find that the absolute krypton concentration of the xenon can be inferred from the mass spectrometry measurements, and we identify krypton signals at concentrations as low as 0.5x10^{-12} mol/mol (Kr/Xe). This technique simplifies the monitoring of krypton backgrounds for WIMP dark matter searches in liquid xenon. 
A. Dobi, C. G. Davis, C. Hall, T. Langford, S. Slutsky, Y. R. Yen
astro-ph.IM astro-ph.CO physics.ins-det