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 parts per thousand over the entire sample size range, with precision only becoming worse than approximately +/- 1 parts per thousand at < 80 pmol. 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-degrees-C, to 1 nmol; in the temperature range of most interest for extraterrestrial samples (600-1200-degrees-C), less-than-or-equal-to 50 pmol 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.