The LC IsoLink interface couples HPLC and IRMS, facilitating sensitive, accurate on-line determination of 13C/12C ratios for compounds insufficiently volatile or too thermally labile to be separated by gas chromatography. The interface preserves the integrity of chromatographic resolution and enables acquisition of isotopic data on an HPLC time scale. Applications include metabolism studies, food and pharmaceutical authentication, doping control, and biogeochemistry research.