  Hydrogen fluoride has been established to be an excellent tracer of molecular
hydrogen in diffuse clouds. In denser environments, however, the HF abundance
has been shown to be approximately two orders of magnitude lower. We present
Herschel/HIFI observations of HF J=1-0 toward two high-mass star formation
sites, NGC6334 I and AFGL 2591. In NGC6334 I the HF line is seen in absorption
in foreground clouds and the source itself, while in AFGL 2591 HF is partially
in emission. We find an HF abundance with respect to H2 of 1.5e-8 in the
diffuse foreground clouds, whereas in the denser parts of NGC6334 I, we derive
a lower limit on the HF abundance of 5e-10. Lower HF abundances in dense clouds
are most likely caused by freeze out of HF molecules onto dust grains in
high-density gas. In AFGL 2591, the view of the hot core is obstructed by
absorption in the massive outflow, in which HF is also very abundant 3.6e-8)
due to the desorption by sputtering. These observations provide further
evidence that the chemistry of interstellar fluorine is controlled by freeze
out onto gas grains.
