We present a detailed chemical composition analysis of 35 red giant stars in the globular cluster M 22. High resolution spectra for this study were obtained at five observatories, and analyzed in a uniform manner. We have determined abundances of representative light proton-capture, α, Fe-peak and neutron-capture element groups. Our aim is to better understand the peculiar chemical enrichment history of this cluster, in which two stellar groups are characterized by a different content in iron, neutron capture elements Y, Zr and Ba, and α element Ca. The principal results of this study are: (i) substantial star-to-star metallicity scatter (- 2.0 ≲ [Fe/H] ≲ -1.6); (ii) enhancement of s-process/r-process neutron-capture abundance ratios in a fraction of giants, positively correlated with metallicity; (iii) sharp separation between the s-process-rich and s-process-poor groups by [La/Eu] ratio; (iv) possible increase of [Cu/Fe] ratios with increasing [Fe/H], suggesting that this element also has a significant s-process component; and (v)presence of Na-O and C-N anticorrelations in both the stellar groups. Based on data collected at: Anglo-Australian Telescope with the University College London Echelle Spectrograph, Apache Point Observatory with the ARC Echelle Spectrograph, Lick Shane 3.0 m Telescope with the Hamilton Echelle Spectrograph, McDonald Smith 2.7 m Telescope with the Robert G. Tull Coudé Spectrograph, and European Southern Observatory with the FLAMES/UVES spectrograph.Tables 3-6 are available in electronic form at http://www.aanda.org