We present a detailed chemical composition analysis of 35 red giant stars in the globular cluster M22 .
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 , \alpha , 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 \alpha element Ca .
The principal results of this study are : ( i ) substantial star-to-star metallicity scatter ( - 2.0 \lesssim [ Fe/H ] \lesssim - 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 .