A sealed-tube method for offline δ¹³ C analysis of CO₂ via a Gas Bench II continuous-flow isotope ratio mass...

The isotopic measurement of environmental sample CO₂ via isotope ratio mass spectrometry (IRMS) can present many analytical challenges. In many offline applications, exceedingly few samples can be prepared per day. In such applications, long-term storage (months) of sample CO₂ is desirable, in order to accumulate enough samples to warrant a day of isotopic measurements. Conversely, traditional sample tube cracker systems for dual-inlet IRMS offer a capacity for only 6-8 tubes and thus limit throughput. Here we present a simple method to alleviate these concerns using a Gas Bench II gas handling device coupled with continuous-flow IRMS.

Sample preparation entails the cryogenic purification and quantification of CO₂ on a vacuum line. Sample CO₂ splits are expanded from a known volume to several sample ports and allowed to isotopically equilibrate (homogenize). Equilibrated CO₂ splits are frozen into 3 mm outer diameter Pyrex break-seals and sealed under vacuum with a torch to a length of 5.5 cm. Sample break-seals are scored, placed into 12 mL Labco Exetainer^® vials, purged with ultrahigh-purity helium, cracked inside the capped helium-flushed vials and subsequently measured via a Gas Bench equipped IRMS instrument using a CTC Analytics PAL autosampler.

Our δ¹³ C results from NIST and internal isotopic standards, measured over a time period of several years, indicate that the sealed-tube method produces accurate δ¹³ C values to a precision of ±0.1‰ for samples containing 10-35 μgC. The tube cracking technique within Exetainer vials has been optimized over a period of 10 years, resulting in decreased sample failure rates from 5-10% to <1%.

This technique offers an alternative method for δ¹³ C analyses of CO₂ where offline isolation and long-term storage are desired. The method features a much higher sample throughput than traditional dual-inlet IRMS cracker setups at similar precision (±0.1‰).