Mass–metallicity relation of Q 2310−3358 derived through the correlation between logarithmic velocity width and metallicity, measured by two unsaturated metal lines, Fe IIλ2374 and Si IIλ1808. The two dashed blue lines show the linear relation fit by Ledoux et al. (2006) based on 70 DLAs in the redshift range 1.7 < z < 4.3, while the solid red line illustrates the linear relation from Møller et al. (2013), based on 110 DLAs spanning 0.11 < z < 5.06.