  We continue our series of papers on intergalactic medium (IGM) tracers using
quasi-stellar objects (QSOs), having examined gamma-ray bursts (GRBs) and
blazars in earlier studies. We have estimated the IGM properties of hydrogen
column density (Nhxigm), temperature and metallicity using XMM-Newton QSO
spectra over a large redshift range, with a collisional ionisation equilibrium
(CIE) model for the ionised plasma. The Nhxigm parameter results were robust
with respect to intrinsic power laws, spectral counts, reflection hump and soft
excess features. There is scope for a luminosity bias given both luminosity and
Nhxigm scale with redshift, but we find this unlikely given the consistent IGM
parameter results across the other tracer types reviewed. The impact of
intervening high column density absorbers was found to be minimal. The Nhxigm
from the QSO sample scales as (1 + z)^1.5+/-0.2. The mean hydrogen density at z
= 0 is n0 = (2.8 +/- 0.3) x 10^-7 cm^-3, the mean IGM temperature over the full
redshift range is log(T/K) = 6.5+/-0.1, and the mean metallicity is [X/H] =
-1.3+/-0.1(Z 0.05). Aggregating with our previous GRB and blazar tracers, we
conclude that we have provided evidence of the IGM contributing substantially
and consistently to the total X-ray absorption seen in the spectra. These
results are based on the necessarily simplistic slab model used for the IGM,
due to the inability of current X-ray data to constrain the IGM redshift
distribution
