Zn transitions, it is impossible to measure its abundance in low-metallicity DLAs, and besides that, the large rest-frame wavelengths of the transitions at 2026 and 2062 [Å]{} make them difficult to measure at high redshift.

Studies of Zn in damped Lyman-$\alpha$ systems with different redshifts have shown that \[Zn/H\] tends to show no correlation with distance and age. Akerman et al. (2005), for example, suggest that \[Zn/H\] in DLAs shows a constant value of \[Zn/H\] = $-$0.88$\pm$0.21 in the redshift range 1.86 $<$ z$_{abs}$ $<$ 3.45. Comparisons of stellar data with abundances in DLAs were carried out by several authors (Pettini et al. 1994; Pettini et al 1997a; Prochaska et al. 2000; Wolfe et al. 2005; Nissen et al. 2007).

In Figs. \[DLA\]a,b, we plot \[Zn/H\] vs. \[Fe/H\] for the present sample and for the DLA samples by Akerman et al. (2005), Cooke et al. (2011, 2013), Kulkarni et al. (2007), and Vladilo et al. (2011). For the Akerman et al. (2005) data, we did not consider the cases with upper limits alone. For the DLA samples of Kulkarni et al. (2007) and Vladilo et al. (2011), a tranformation \[Fe/H\] vs. redshift from the literature (Pei & Fall 1995) was 