Until a few years ago globular clusters (GCs) were considered the prototypes of simple stellar populations, composed of coeval stars with same chemical composition. Indeed, their color-magnitude diagrams (CMDs), including those diagrams obtained from high-precision Hubble Space Telescope (HST) photometry, were similar to single isochrones. Thanks to innovative techniques of photometric data reduction and new color combinations sensitive to chemical differences among stars, it has been discovered that GCs are much more complex than previously thought: all the evolutionary phases of their CMDs contain indeed multiple distinct sequences that correspond to stellar populations with different content of light-elements and helium. GCs therefore host multiple stellar populations, including a first population with the same chemical composition of halo field stars at the same metallicity, and one or more second populations enhanced in He, N, and Na and depleted in C and O. One of the most unexpected discoveries, mostly based on evolved red giant branch (RGB) stars of more than 50 GCs, is that the first population is not chemically homogeneous. With the aim of shedding new light on this phenomenon, we combined multi-band HST photometry of GCs and synthetic spectra analysis techniques, to investigate for the first time chemical variations among unevolved main sequence (MS) stars. We present here the results of NGC 6362 and discuss the implications on the chemical composition of the pristine material from which they formed.

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