Adaptation of continuous-flow cavity ring-down spectroscopy for batch analysis of δ13C of CO2 and comparison with isotope ratio mass spectrometry

Measurements of delta C-13 in CO2 have traditionally relied on samples stored in sealed vessels and subsequently analyzed using magnetic sector isotope ratio mass spectrometry (IRMS), an accurate but expensive and high-maintenance analytical method. Recent developments in optical spectroscopy have yielded instruments that can measure delta(CO2)-C-13 in continuous streams of air with precision and accuracy approaching those of IRMS, but at a fraction of the cost. However, continuous sampling is unsuited for certain applications, creating a need for conversion of these instruments for batch operation. Here, we present a flask (syringe) adaptor that allows the collection and storage of small aliquots (20-30mL air) for injection into the cavity ring-down spectroscopy (CRDS) instrument. We demonstrate that the adaptor's precision is similar to that of traditional IRMS (standard deviation of 0.3 parts per thousand for 385ppm CO2 standard gas). In addition, the concentration precision (+/- 0.3% of sample concentration) was higher for CRDS than for IRMS (+/- 7% of sample concentration). Using the adaptor in conjunction with CRDS, we sampled soil chambers and found that soil-respired delta C-13 varied between two different locations in a pinon-juniper woodland. In a second experiment, we found no significant discrimination between the respiration of a small beetle (similar to 5mm) and its diet. Our work shows that the CRDS system is flexible enough to be used for the analysis of batch samples as well as for continuous sampling. This flexibility broadens the range of applications for which CRDS has the potential to replace magnetic sector IRMS. Copyright (C) 2011 John Wiley & Sons, Ltd.