Aims .
In this paper we present the chemical abundance analysis from high resolution UVES spectra of seventeen bright giant stars of the Globular Cluster ( GC ) M 22 . Results .
We obtained an average iron abundance of [ Fe / H ] = -1.76 \pm 0.02 ( internal errors only ) and an \alpha enhancement of 0.36 \pm 0.04 ( internal errors only ) .
Na and O , and Al and O follow the well known anti-correlation found in many other GCs .
We identified two groups of stars with significantly different abundances of the s -process elements Y , Zr and Ba .
The relative numbers of the two group members are very similar to the ratio of the stars in the two SGBs of M 22 recently found by Piotto ( 2009 ) .
Y and Ba abundances do not correlate with Na , O and Al .
The s -element rich stars are also richer in iron and have higher Ca abundances . The results from high resolution spectra have been further confirmed by lower resolution GIRAFFE spectra of fourteen additional M 22 stars .
GIRAFFE spectra show also that the Eu – a pure r -process element – abundance is not related to the iron content .
We discuss the chemical abundance pattern of M 22 stars in the context of the multiple stellar populations in GC scenario .