A high-precision mass spectrometer for stable carbon isotope analysis at the nanogram level

Details are presented of a double-collector static mass spectrometer that can determine precise stable carbon isotope ratios for samples of the order of 10 -8 g carbon as CD 4 . Measurements are performed by analysis of the ion-beam intensities at m/z=20 ( 12 CD 4 + )and 21 ( 13 CD 4 + ). The instrument is capable of long-term reproducibility (the 20/21 ratio of 23 consecutive aliquots of standard gas analysed over the course of a day varies by only +or-0.16 per mille); its ability to provide comparative isotopic measurements ( delta values) has been assessed by a zero-enrichment check which affords a value of -0.012+or-0.25 per mille.