Line area ratio for the pairs of isotopologues (left panel). The ratios expected at the galactocentric distance of G328.2551-0.5321 from galactocentric variation measurements are indicated (see Table 6 for references). The error bars represent the errors on the line area given by the Gaussian fitting procedure from CLASS. Right panel: ratio of the measured isotopie ratio obtained from the LTE modelling of the column densities and the ratios expected at the galactocentric distance of G328.2551-0.5321 from galactocentric variation measurements. The error bars represent a 20% error on the column densities.