The Tiger XT detects from 1 ppb to 20,000 ppm and supports 10.6, 11.7, and 9.8 eV lamps.
The Tiger XTL detects from 0.1 ppm to 5,000 ppm and only supports a 10.6 eV lamp.
The Tiger XT is aimed at trace detection (ppb), while the Tiger XTL is designed for standard industrial applications in ppm.