We report high-precision isotope carbon dioxide measurements, made before and after ion source modification to gas isotope ratio mass spectrometry (IRMS) instruments. Measurement protocols were designed to explore the effects of ion surce material substitution, source conductance, inlet pressure, electron emission, acceleration potential, and inlet changeover equilibration time. After modificiation of the IRMS instruments at the National Institute of Standards and Technology (NIST) and the Max-Planck-Institute for Chemistry (MPI-Mainz), immediate changes were observed. At NIST, measurements were no longer sensitive to inlet equilibration times greater than 15 s, and different settings of ion source conductance resulted in delta- 13C shifts of about 0.004% per 1% measurement difference between sample and reference, a five-fold improvement. No significatn changes in machine performance were observed after a month of use. After a year, performance had degraded slightly, but was controlled by ion source cleaning and the use of low-energy ion acceleration to minimize sputtering. At MPI-Mainz, results were very similar. We report cross-contamination coefficients measured since 1996, and discuss the role of adsorption, ion implantation, and sputtering on cross contamination in mass spectrometry systems. We recommend that users of high-precision IRMS instruments test for and minimize the effects described.
Citation: Rapid Communications in Mass Spectrometry
Pub Type: Journals
carbon dioxide, differential isotope ratio, inlet change, inlet equilibration time, ion source conductance, isotopic compositions, reference materials