  We study the extent to which one can distinguish primordial non--Gaussianity
(NG) arising from adiabatic and isocurvature perturbations. We make a joint
analysis of different NG models based on various inflationary scenarios:
local-type and equilateral-type NG from adiabatic perturbations and local-type
and quadratic-type NG from isocurvature perturbations together with a
foreground contamination by point sources. We separate the Fisher information
of the bispectrum of CMB temperature and polarization maps by l for the skew
spectrum estimator introduced by Munshi & Heavens (2009) to study the scale
dependence of the signal-to-noise ratio of different NG components and their
correlations. We find that the adiabatic and the isocurvature modes are
strongly correlated, though the phase difference of acoustic oscillations helps
to distinguish them. The correlation between local-and equilateral-type is
weak, but the two isocurvature modes are too strongly correlated to be
discriminated. Point source contamination, to the extent to which it can be
regarded as white noise, can be almost completely separated from the primordial
components for l>100. Including correlations among the different components, we
find that the errors of the NG parameters increase by 20-30% for the WMAP
5-year observation, but 5% for Planck observations.
