Halite fluid inclusions have recently been used to place direct constraints on the composition of Earth's evolving atmosphere. By crushing these minerals, the gases can be released to and quantified by mass spectrometers. New constraints on carbon dioxide concentrations will be extremely valuable for deciphering Earth's climatic history, but verification that the CO₂ is indeed atmospheric is necessary. Here we utilize a dual inlet isotope ratio mass spectrometer to measure the δ¹³C of the inclusion CO₂ to evaluate the origin of the trapped CO₂, and determine what corrections are necessary to the data.