  We present a survey of six low to moderate redshift quasars with Chandra and
XMM-Newton. The primary goal is to search for the narrow X-ray absorption lines
produced by highly ionized metals in the Warm-Hot Intergalactic Medium. All the
X-ray spectra can be fitted by a power law with neutral hydrogen absorption
method. The residuals that may caused by additional emission mechanisms or
calibration uncertainties are taken account by polynomial in order to search
for narrow absorption features. No real absorption line is detected at above
3-sigma level in all the spectra. We discuss the implications of the lack of
absorption lines for our understanding of the baryon content of the universe
and metallicity of the intergalactic medium (IGM). We find that the
non-detection of X-ray absorption lines indicates that the metal abundance of
the IGM should be smaller than ~0.3 solar abundance. We also discuss
implications of the non-detection of any local (z ~ 0) X-ray absorption
associated with the ISM, Galactic halo or local group, such as has been seen
along several other lines of sight (LOS). By comparing a pair of LOSs we
estimate a lower limit on the hydrogen number density for the (z ~ 0) 3C 273
absorber of n_H >= 4e-3 cm^-3.
