in complete absence of dust. Since DLA show some amount of dust, a moderate enhancement of Si over Fe cannot be considered as a clear-cut evidence of a halo-like pattern. Moreover, values of \[Si/Fe\]$\ge$0.6, that would be expected if differential depletion and intrinsic $\alpha$ enhancement are present, have not yet been observed.

Nitrogen and oxygen abundances
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Nitrogen has been detected in the DLA at z=3.39 towards QSO 0000-26, with \[N/H\] = $-$2.77 $\pm 0.17$ (Vladilo et al. 1995 and Molaro et al. 1995) and in the DLA at $z$=1.78 towards MC3 1331+170 with \[N/H\]=-2.7 (Green et al. 1995). A significant upper limit has been derived by Pettini et al. (1995) in the DLA systems towards QSO 2348-147 NI$<$-3.15. By means of co-addition technique of the lines of the 1200 Å triplet Lipman (1995) achieved a marginal detection of NI for the main component of the DLA towards QSO 2344+124. By using the data published by Fan and Tytler (1994), Lipman derived also an upper limit of \[N/H\] $<$ -3.26. These measurements show that a real dispersion in the nitrogen abundances may be present among the DLAs.

In order to understand the nucleosynthetic origin of nitrogen the ideal would be 