We present a new method to estimate the fraction of stars with chemical composition of first and second ( s ) generation ( s ) currently hosted in Galactic globular clusters ( GCs ) .
We compare cluster and field stars of similar metallicity in the [ Fe/H ] - [ Na/H ] plane .
Since the phenomenon of multiple populations is only restricted to the cluster environment , the number of GC stars whose location coincides with that of field stars provides the fraction of first generation stars in that cluster .
By exclusion , the fraction of second generation stars is derived .
We assembled a dataset of 1891 field stars of the thin disk , thick disk , and halo of the Milky Way in the metallicity range -3.15 \leq [ Fe/H ] \leq + 0.48 dex and with Na abundance from high resolution spectra .
They are mostly dwarfs , but include also giants .
Considering only the range in metallicity spanned by most GCs extensively studied for the Na-O anticorrelation ( -2.36 \leq [ Fe/H ] \leq - 0.33 dex ) , we have 804 stars .
The total sample is homogeneized by offsets in [ Fe/H ] and [ Na/H ] with respect to a reference sample using the same line list and NLTE correction for Na adopted in a recent extensive survey of GC stars .
This fully accounts for offsets among analyses due to different temperature scales , line lists , adopted ( or neglected ) corrections for departures from LTE .
We illustrate our method estimating the fraction of first and second generation stars in the well studied GC NGC 6752 .
As a by-product , the comparison of [ Na/H ] values in GC and field stars suggests that at least two classes of old stellar systems probably contributed to the halo assembly : one group with characteristics similar to the currently existing GCs , and the other more similar to the present-day dwarf satellite galaxies .