Synthetic fits of the observed spectra of the M 22 RGB stars #III-15, #C, and #III-52 (from top to bottom). The left panels show a rather large spectral domain including the weak HF-transition together with two strong CO lines, confirming the right radial velocity correction adopted for the fits. A zoom of the left panels around the HF-line is shown in the right panels. Three different fluorine abundances spanning a wide range of possible values have been adopted for the synthetic spectra (see the labels in each right-hand panels).