to the solar abundances of Anders & Grevesse (1989), with the exception of $\log (Fe/H)_{\sun}$=$-4.52$ taken from Hannaford et al. (1992) (\[M/H\] $\equiv$ log(M/H)-log(M/H)$_{\sun}$). The abundances in the Tables are derived from column density ratios of different atomic and ionic species in the gas phase, and can be translated to elemental abundances only if ionization effects and dust depletion are negligible or can be accounted for. In the following we discuss shortly these two effects.

     QSO       $z_{\rm abs}$  N(HI)      \[N/H\]                        \[O/H\]                   \[Si/H\]                  \[S/H\]                   \[Fe/H\]               Refs.
  ----------- --------------- ------- ------------------------------ ------------------------- ------------------------- ------------------------- ------------------------- ---------
                                                                                                                                                                             
  0000-26         3.3901      21.30   $-2.77^{+0.17}_{-0.17}$        $-3.13^{+0.17}_{-0.17}$   $-2.48^{+0.19}_{-0.19}$       —                     $-2.38^{+0.16}_{-0.16}$     1
                                                                                                                                                                             
  1331+1700       1.7765      21.18   $-2.73^{+0.11}_{-0.11}$        $-2.81^{+0.21}_{-0.30}$       —                     $-1.35^{+0.11}_{-0.11}$   $-2.25^{+0.11}_{-0.12}$     2$^a$
                                                                                                                                                                             
  2348-147        2.2794      20.57   $<-3.15$                       $-2.13^{+2.26}_{-0.58}$   $-1.97^{+0.13}_{-0.10}$   $-1.91^{+0.14}_{-0.16}$   $-2.35^{+0.22}_{-0.10}$     3
                                                                                                                                                                             
  2344+124        2.5379      20.43   $-3.00^{+0.12}_{-0.22}$ $^b$   $-2.11^{+2.5}_{-0.36}$    $-1.72^{+0.27}_{-0.13}$   $<-1.20$                  $-1.85^c$                   4
                                                                                                                                                                             
  1946+7658       2.8443      20.30   $<-3.26$ $^d$                  $-2.65^{+0.15}_{-0.15}$   $-2.16^{+0.10}_{-0.10}$   $<-0.89$                  $-2.41^{+0.10}_{-0.10}$     5$^e$
                                                                                                                                                                             

0.4 truecm

References for Table 1\
(1) Molaro et al. (1995) ; (2) Green et al. (1995) ; (3) Pettini et al.(1995);\
(4) Lipman, thesis (1995) ; (5) Lu et al. (1995a)

0.4 truecm

$a$ : Hydrogen column density from Pettini et al. (1994); metal abundance error bars include propagation of the N(HI) error.

$b$ : Nitrogen value derived 