Optical feedback cavity-enhanced absorption spectroscopy for in situ measurements of the ratio 13 C : 12 Ci n CO 2

We report on the design and laboratory performance of a portable infrared absorption spectrometer for the measure- ment of the isotopic ratio 13 C : 12 Ci n CO 2. The design relies on optical feedback cavity-enhanced absorption spectroscopy in the 2 µm spectral region to achieve optimal performance at am- bient CO2 concentrations. The prototype instrument measures δ 13 C, relative to a standard calibration bottle, with a precision of ±0.7‰ for a 20-s integration time and with an automatic recal- ibration every 6 min. The absolute accuracy obtained is 0.9‰. The principal performance limitations are discussed along with improvements currently being implemented for the second gen- eration instrument.