ducted to estimate the integrated spectral line intensity of the 2ν 1 band of HC 3 N. HCN was used as the spectroscopic reference species, with quantitative temperaturedependent line intensities (S ij ) taken from the HITRAN database ). The gas from a mixed, dilute cylinder prepared with both HCN and HC 3 N was flowed into the buffer-gas cell held at 20 K to enable a relative line intensity measurement. Flowing the two gases simultaneously from the same source and measuring their spectra in the same experiment enabled the cancellation of many instrument parameters, such as uncertainty in the absorber pathlength and fluid dynamics.