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With the emergence of wide-spread application of new optical techniques to monitor δ¹³C in atmospheric CO₂ there is a growing need to ensure well-calibrated measurements. We characterized one commonly available instrument, a cavity ring-down spectrometer (CRDS) system used for continuous in situ monitoring of atmospheric ¹³CO₂. We found no dependency of δ¹³C on the CO₂ concentration in the range of 303–437 ppm. We designed a calibration scheme according to the diagnosed instrumental drifts and established a quality assurance protocol. We find that the repeatability (1-σ) of measurements is 0.25‰ for 10 min and 0.15‰ for 20 min integrated averages, respectively. Due to a spectral overlap, our instrument displays a cross-sensitivity to CH₄ of 0.42 ± 0.024‰ ppm⁻¹. Our ongoing target measurements yield standard deviations of δ¹³C from 0.22‰ to 0.28‰ for 10 min averages. We furthermore estimate the reproducibility of our system for ambient air samples from weekly measurements of a long-term target gas to be 0.18‰. We find only a minuscule offset of 0.002 ± 0.025‰ between the CRDS and Environment Canada's isotope ratio mass spectrometer (IRMS) results for four target gases used over the course of one year.