A question for the GC IRMS community about methane conditioning of an Al2O3 TC reactor for CSHIA. I set up the GC TC system using an empty ceramic tube heated at 1450°C as the pyrolysis reactor. I added a methane line in backflush on the Aux gas line on the GC II/III interface in order to condition the reactor with black carbon prior to the analysis. I tried to do a 10 sec BF, which is not enough to get close enough to the real value of the standard testes (5 alpha androstane from Indiana university dD=-297.3). After the second 10 sec BF i got an increase of my mass 28,29,30 baseline (from 7 to 500 mV) which I'm interpreting as residual pyrolysis by-product (CO) of my conditioning considering the fact that my Ar, O2 and H20 are still nice and low). This baseline increase is bothering me because I don't think I can trust the dD value given for my standard peak now. Today (2 days after the conditioning) the 28,29,30 baseline is down to roughly 70 mV. Would anyone be able to tell me how much methane is needed in order to have a good carbon coating without overloading the reactor?
In my lab we run compound specific 2H isotope analyses on a Thermo Delta V plus coupled to a GC-Isolink. Though the system is set up for methane conditioning I found the entire process a bit too cumbersome. It is much more convenient and easier to inject small volumes (ca. 0.1 microL) of n-hexane or n-heptane on to the GC (isothermal at 70C) and allow this solvent peak to enter the thermal-conversion (TC) reactor. Of course, during this process the open-split connection to the IRMS should be disconnected (diverted) to prevent the solvent peak (or what's left of it) to enter the ion source. Typically two or three such injections will "carbonise" the TC reactor tube sufficiently for 15 to 20 samples to be run in triplicate.
Thanks for the trick. I ended up doing 2 to 3 consecutives 10 sec of CH4 backflush followed by an hour of He backflush in order to have a conditioning lasting for at least 100 injections. I prefere the CH4 conditioning because the amount of time needed to stabilize the background seems to be shorter and it seems to last for a longer period of time. (at least with the type of sample that i was injecting, probably contributing to the carbon load in the ceramic tube too)
btw, our GC-TC is running on our Thermo Delta plus XL, and I have to say that I'm pretty impressed with the range of linearity and sensitivity obtained for aromatic compounds, stay tuned for the upcoming paper. Cheers
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