The technique of Atom Trap Trace Analysis (ATTA) was developed at Argonne National Laboratory for use in radioisotope dating and nuclear monitoring, but it can also be applied to the measurement of trace amounts of krypton in samples of other rare gases. This is a critical problem for astrophysical particle detectors such as the XENON dark matter search and the XMASS and CLEAN neutrino detectors, where the beta decay of 85Kr (85Kr/Kr ~ 10-11, τ1/2 ~11 yrs) is an important source of background. These detectors require either xenon or neon gas with natural krypton contamination at the level of a few parts in 1010 (XENON) or even 1015 (CLEAN, XMASS).
As Kr is chemically inert, it is very difficult to separate from other rare gases, and it is very difficult and time-consuming to measure Kr contamination at the necessary level. The ATTA technique may provide a fast and accurate way of measuring the Kr content of other rare gases by trapping and detecting single Kr atoms in a sample of other gas. A simple estimate suggests that an ATTA system could measure krypton at the 1014 level in only 3 hours of integration.
For a more detailed explanation of the technique, there is a preprint describing the technique. I recently gave a talk on ATTA at the LRT2004 workshop, and the PowerPoint slides are online (scroll down to Monday morning).