Knowing atmospheric 13CO2/12CO2 ratios precisely is important to understanding biogenic and anthroprogenic sources and sinks for carbon. Currently available field deployable instrumentation have unfavourable attributes such as high power requirements, are extremely expensive, are too large, are not proven or designed to run continuously or unattended for extended periods, and/or are unable to properly compensate for nature pressure, temperature and moisture variations. Southwest Sciences proposes to develop a high precision isotopic carbon dioxide measurement system that nullifies interferences from pressure, temperature and moisture, and that has characteristics compatible with field deployable instrumentation. This instrument would be fully autonomous, requires no consumable and would not need periodic maintenance. In Phase 1, we will build a simplified single-pass spectrometer and demonstrate the ultimate achievable precision under ideal conditions, operating parameters, and Allan variance measurements. In Phase 2, we will leverage Southwest Science's extensive experience with multi-pass cells and incorporate additional temperature and pressure compensation techniques for operation under non-ideal conditions. At the end of Phase 1, we expect this instrument to be at TRL-4.