An analytical system for determining delta17O in CO2 using continuous flow-isotope ratio MS.

We developed a simple measurement system for delta17O in nanomole quantities of CO2 using continuous flow isotope ratio mass spectrometry (CF-IRMS). The analytical system consisted of a sample injection system, a helium-purged CO2 purification line, a capillary GC, a combustion unit, and CF-IRMS. A unique feature of the system is that we use molecular CO2 to determine the isotopic compositions including delta17O. The delta17O of CO2 in a sample is calculated from the mass ratios of both 45/44 and 46/44 of two different kinds of CO2, which have been purified quantitatively from different aliquots of a sample. While one aliquot (rCO2) flows into IRMS directly, the other (eCO2) flows through a CuO unit (900 degrees C) prior to injection into IRMS, to exchange oxygen atoms in the sample CO2 molecules with those in CuO for which we can assume Delta17O = 0. In our system, we introduce both rCO2 and eCO2 alternately to IRMS repeatedly by using an automatic multianalytical system to improve analytical precision statistically. The standard deviation of 0.35 per thousand for Delta17O can be realized using as little as 8.7 nmol CO2 in a approximately 3-h analysis. Based on this system, we have quantified delta17O in the stratospheric CO2 over Japan.