In panel a) the mass − metallicity relation derived by Tremonti et al. (2004) (dashed line) is plotted together with data for galaxies in our sample with a known SDSS metallicity, given in Table 1. In panels b) and c) the mass − metallicity relation has been scaled so that its peak reproduces the average dust-to-gas ratio for the M-sample. In panel b) is used to compute M_gas and the dot-dashed line is the best fit linear relation to galaxies in the M- and S-sample. In panel c) is used in computing M_tot.