Velocity shift (with respect to lines with the standard isotopic ratio) as a function of ξ = (²⁵Mg + ²⁶Mg)/²⁴Mg assuming r = ²⁶Mg/²⁵Mg = 1 for mock ESPRESSO data. The solid blue line corresponds to the mean value for 100 mock spectra for each input ξ value, and the dark and light-blue regions represent the regions containing 68.3% and 95.5% of the values, respectively. The pink line corresponds to a noiseless mock spectrum. The observed velocity shift and associated 1σ uncertainty for the system at z = 0.45206 are shown as orange horizontal solid and dotted lines, respectively.