  We study the distribution of long Gamma Ray Bursts in the Ep-Eiso and in the
Ep,obs-Fluence planes through an updated sample of 76 bursts, with measured
redshift and spectral parameters, detected up to September 2007. We confirm the
existence of a strong rest frame correlation Ep ~ Eiso^0.54+-0.01. Contrary to
previous studies, no sign of evolution with redshift of the Ep-Eiso correlation
(either its slope and normalisation) is found. The 76 bursts define a strong
Ep,obs-Fluence correlation in the observer frame (Ep,obs ~ F^0.32+-0.05) with
redshifts evenly distributed along this correlation. We study possible
instrumental selection effects in the observer frame Ep,obs-Fluence plane. In
particular, we concentrate on the minimum peak flux necessary to trigger a
given GRB detector (trigger threshold) and the minimum fluence a burst must
have to determine the value of Ep,obs (spectral analysis threshold). We find
that the latter dominates in the Ep,obs-Fluence plane over the former. Our
analysis shows, however, that these instrumental selection effects do not
dominate for bursts detected before the launch of the Swift satellite, while
the spectral analysis threshold is the dominant truncation effect of the Swift
GRB sample (27 out of 76 events). This suggests that the Ep,obs-Fluence
correlation defined by the pre--Swift sample could be affected by other, still
not understood, selection effects. Besides we caution about the conclusions on
the existence of the Ep,obs-Fluence correlation based on our Swift sample
alone.
