  We present a detailed analysis of the ionisation state and heavy element
abundances in the Intergalactic Medium (IGM). The CIV doublet is shown by 30 %
of the 182 selected optically thin \lya clouds in 10 QSO lines of sight. Direct
metallicity calculations have been performed on individual systems with
detected CIV and SiIV (10% of the sample) varying the UV photoionising source,
cloud density and size and silicon relative abundance. The best solutions for
carbon content in this subsample (redshift coverage $z=2.6 - 3.8$) span between
1/6 and 1/300 of the solar value with no evidence of redshift evolution in both
the metallicity and the ionising source. Global properties of the whole sample
indicate that the metallicity in \lya clouds with CIV and SiIV is not typical
of the IGM. The redshift evolution of the UVB is one of the possible sources of
the observed SiIV/CIV trend presented by Cowie and collaborators during this
meeting. Future detection of heavy elements in lower HI column density ($\log
N_{HI} < 14.5$) \lya clouds relies on the presence of OVI and NV at $z=1-2.5$.
