$\le$ 0.2 dex and S essentially undepleted (Jenkins 1987). Reddening measurements of QSO with DLA systems suggest a dust-to-gas ratio in the DLA of about 10% of that in our Galaxy (Fall and Pei 1989). In the survey of ZnII and CrII in DLA by Pettini et al. (1994) chromium is typically one dex below zinc, while in the Galaxy is about two dex, showing the presence of dust and a reduced dust depletion in the DLA systems. If some dust were present this would alter the abundance determinations of refractory elements, such as Fe, Si and Al. Much more reliable are the abundances of the non-refractory elements, such as Zn, S, O and N.

$\alpha$ versus iron-peak elements
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Of particular importance for understanding galactic chemical evolution is the comparison between $\alpha$ and iron peak elements. In particular, the ratio between sulphur and zinc is an important diagnostic tool. Both elements show little affinity with dust and their ratio is also safe against possible contributions from HII regions, since these essentially cancel out. Considering that S and Zn have different nucleosynthetic origin with S mainly a product of type II SN and Zn of type Ia SN (Matteucci et 