Application of high-sensitivity carbon isotope techniques— a question of blanks

The performance of a new carbon stable isotope static mass spectrometer over the sample size range 10 pmol to 1 nmol is reviewed with reference to standard data gathered over a year of operation. Specimens of known isotopic composition and zero enrichment tests suggest that the instrument is accurate to within±0.5‰ over the entire sample size range, with precision only becoming worse than∼±;1‰at< 80pmol. Efforts to obtain “blanks” compatible with the above performance are also reviewed. A procedure is available which controls the total blank in a stepped combustion, from room temperature to 1200°C, to 1 nmol; in the temperature range of most inserest for extraterrestrial samples (600–1200°C),⩽50pmol can be achieved. However, a real benefit of the new instrument is that isotopic compositions of the blanks can be accurately obtained, so allowing a proper assessment of its causes and a correction of sample data if desired.