Fusion facilities like ITER and DEMO will circulate several kilograms tritium and deuterium per day in their fuel cycle. For the separation of the hydrogen isotopologues the Isotope Separation System (ISS), based on cryogenic distillation, was developed at Tritium Laboratory Karlsruhe (TLK). One challenge is to find and develop an in situ and real time method to analyse the isotopologic composition of the column content. Calibration tests with IR absorption spectroscopy (FTIR) with chemically equilibrated samples have been performed at the Tritium absorption IR Spectroscopy Experiment (TApIR). From this previous work and from literature, it is known that the dependence between IR absorbance and the concentrations is non-linear. This makes it impossible to extrapolate the calibration from equilibrium to non-equilibrium samples. This work shows a full D2, H2, and HD calibration with samples in and off the high temperature. This enables us now to measure composition of inactive liquid hydrogen samples with an accuracy of better than 5%. In addition, one of the main challenges on the way to a calibration with tritiated mixtures is shown, the IR absorbance at molecular dimers, which tremendously increases the complexity of IR absorption spectra.