The iron dispersion of the globular cluster M 2, revised
  M 2 has been claimed to posses three distinct stellar components that are
enhanced in iron relative to each other. We use equivalent width measurements
from 14 red giant branch stars from which Yong et al. detect a $\sim$0.8 dex
wide, trimodal iron distribution to redetermine the metallicity of the cluster.
In contrast to Yong et al., which derive atmospheric parameters following only
the classical spectroscopic approach, we perform the chemical analysis using
three different methods to constrain effective temperatures and surface
gravities. When atmospheric parameters are derived spectroscopically, we
measure a trimodal metallicity distribution, that well resembles that by Yong
et al. We find that the metallicity distribution from Fe II lines strongly
differs from the distribution obtained from Fe I features when photometric
gravities are adopted. The Fe I distribution mimics the metallicity
distribution obtained using spectroscopic parameters, while the Fe II shows the
presence of only two stellar groups with metallicity [Fe/H]$\simeq$-1.5 and
-1.1 dex, which are internally homogeneous in iron. This finding, when coupled
with the high-resolution photometric evidence, demonstrates that M 2 is
composed by a dominant population ($\sim$99%) homogeneous in iron and a
minority component ($\sim$1%) enriched in iron with respect to the main cluster
population.
