  We have developed an atom trap trace analysis (ATTA) system to measure Kr in
Xe at the part per trillion (ppt) level, a prerequisite for the sensitivity
achievable with liquid xenon dark matter detectors beyond the current
generation. Since Ar and Kr have similar laser cooling wavelengths, the
apparatus has been tested with Ar to avoid contamination prior to measuring Xe
samples. A radio-frequency (RF) plasma discharge generates a beam of metastable
atoms which is optically collimated, slowed, and trapped using standard
magneto-optical techniques. Based on the measured overall system efficiency of
$1.2 \times 10^{-8}$ (detection mode) we expect the ATTA system to reach the
design goal sensitivity to ppt concentrations of Kr in Xe in $<2$ hours.
