1998A&A...337..246L

We have measured galactic λ3mm absorption spectra of HCO⁺, HCN, HNC, and CS toward compact extragalactic continuum sources in order to derive the isotopic abundance ratios ¹²C/¹³C, ¹⁴N/¹⁵N, ¹⁶O/¹⁸O, and ³²S/³⁴S in local diffuse clouds. For carbon, our data confirm recent results for the local ISM: we find ¹²C/¹³C=59±2. For nitrogen, we find ¹⁴N/¹⁵N=237(-21,+27) consistent with the Solar value of 270, but substantially smaller than the values inferred from HCN emission in dense clouds. For sulfur we find ³²S/³⁴S ≃19±8 consistent with the Solar value of 23. We also find one striking individual anomaly: toward 3C111 (B0415+379), H¹²CN/^H13^CN=170±50 in one kinematic component. We attribute this to the fractionation of ¹³C into ¹³CO which may be so great that HCN is starved for ¹³C.