Deuterium enhancement of monodeuterated species has been recognized for more than 30 years as a result of chemical fractionation that results from the difference in zero-point energies of deuterated and hydrogenated molecules. The key reaction is the deuteron exchange in the reaction between HD, the reservoir of deuterium in dark interstellar clouds, and the H3+ molecular ion, leading to the production of H2D+ molecule, and the low temperature in dark interstellar clouds favours this production. Furthermore, the presence of multiply deuterated species have incited our group to proceed further and consider the subsequent reaction of H2D+ with HD, leading to D2H+, which can further react with HD to produce D3+. In pre-stellar cores, where CO was found to be depleted, this production should be increased as CO would normally destroy H3+. The first model including D2H+ and D3+ predicted that these molecules should be as abundant as H2D+. The first detection of the D2H+ was made possible by the recent laboratory measurement for the frequency of the fundamental line of para-D2H+. Here, we present observations of H2D+ and D2H+ towards a sample of dark clouds and pre-stellar cores and show how the distribution of ortho-H2D+ (1(1,0)-1(1,1)) can trace the deuterium factory in pre-stellar cores. We also present how future instrumentation will improve our knowledge concerning the deuterium enhancement of H3+.
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