High-precision stable isotope analysis of <5 μg CaCO₃ samples by continuous-flow mass spectrometry

Rationale: Oxygen (δ¹⁸O) and carbon (δ¹³C) isotope analysis of foraminifera and other CaCO₃ samples has been a key technique for paleoceanographical and paleoclimatological research for more than 60 years. There is ongoing demand for the analysis of ever smaller CaCO₃ samples, driven, for example, by the desire to analyse single specimen planktic foraminifera, or small samples of tooth enamel. Methods: We present a continuous-flow mass spectrometric technique that uses cryo-focusing of sample CO₂ to analyse CaCO₃ samples in a weight range between 10 and 3 μg. These are considerably lower sample weights than achievable on most currently available standard instrumentation. The technique is automated, so that sample throughput lies at >60 samples per day. The method involves an on-line vial-flushing routine designed to remove machine drift due to blank CO₂ build-up in the sample vials. Results: In a series of experiments the effect of blank CO₂ build-up is quantified, and outgassing from the chlorobutyl septa identified as the source. An improved flushing routine together with the use of a cryo-focusing step in the analysis is demonstrated to provide the analytical stability and sensitivity to analyse CaCO₃ samples in a weight range between 10 and 3 μg at ≤0.1‰ precision (1σ) for both δ¹⁸O and δ¹³C values. The technique yields similarly precise results for the analysis of the structural carbonate fraction of small tooth enamel samples. Conclusions: This study demonstrates that high-precision oxygen and carbon isotope analysis is possible on CaCO₃ samples smaller than 5 μg by use of a continuous-flow isotope technique. Of key importance are (1) the application of a cold trap that drastically reduces sample gas loss, and (2) a modified flushing regime that eliminates increasing background CO₂ build-up in sample vials during longer automated sample runs.