Molecular absorption lines in the line of sight to distant quasars are an extremely powerful tool to probe the dense interstellar medium and its chemical composition in intervening galaxies from low to high redshifts. The absorption line measurements of different isotopomers even allow us to study isotopic ratios, which can be interpreted as the signature of past nucleosynthesis activity, and put some constraints on the chemical evolution models.

In this paper, we present the study of molecular absorption lines in front of the quasar PKS 1830-211. The absorption is due to an intervening galaxy at $z=0.89$ which is identified as a nearly face-on spiral galaxy. We have carried out a survey of absorption lines of various HCO$^+$, HCN, HNC, and CS isotopomers with the Plateau de Bure Interferometer and derived for the first time the C, N, O, and S isotopic ratios in such a distant object.

This $z=0.89$ absorption system offers an unique opportunity to study the chemical composition in the disk of a spiral galaxy only a few Gyr old. Our results show significantly different isotopic ratios as compared to those measured in the Solar System or in the local ISM, indicating a poorly enhanced abundance of material processed by intermediate and low mass stars.