Measurement of carbon stable isotopes at the nanomole level: a static mass spectrometer and sample preparation technique

A triple collector static vacuum mass spectrometer is described for the determination of carbon isotope ratios from 1*10 -9 mol samples of carbon dioxide gas. The instrument is capable of providing a differential isotopic composition ( delta 13 C value) with a precision (1 sigma ) better than 1 0 / 00 /; furthermore, the long-term stability, assessed by an extended zero-enrichment test, is not worse than +or-1.3 0 / 00 . Because of abundance sensitivity problems, caused by working at high source pressure, the best performance is obtained using a reference gas which is similar in isotopic composition to the sample under investigation. A low blank, stepped heating procedure is outlined for use in preparing samples for study in the instrument. Some examples of problems to which the machine has been applied are given.