This method describes how we use a modified Costech EA coupled to a Finnigan MAT 253 isotope ratio mass spectrometer to measure injected gas samples for δ13C and δ18O of CO2. A septum port is placed downstream of the combustion / reduction columns of the EA but before the desiccant trap. Injected sample gas passes through a desiccant column, a GC column, the conflo II and into the IRMS. This method has been used for demonstration and exploratory purposes. Two standards are available. Both are approximately 10% CO2. One is atmospheric CO2 that was concentrated from the lab, expanded into a cylinder that was subsequently topped off with helium. The other cylinder is from Praxair and is foosil fuel CO2 in air.
- At least a couple hours before starting injections, turn GC temperature to 90°C. Check the condition of injection port septum, perchlorate trap and change accordingly. Make sure the switch is turned clockwise for injections. (Completing a couple hours before will ensure that when the GC is to temperature and that backgrounds will be down for immediate injections)
- Within the acquisition window, switch the gas to N2 and have dilution OFF.
- With dilution OFF, watch the N2 (mass 28 and 29) levels. They should be at or below 10 mV.
- Fill out Shrek_DailyLog.xlsx a. Set the gas to CO2 and record background values b.Set the middle cup to mass 18 (water), do a peak center on mass 18, and record the value. c. Set the middle cup to mass 40 (argon), do a peak center on mass 40, and record the value.
- Enter all your samples into a new sequence table. There are six injections for each run, and each run takes approximately 30 minutes so plan accordingly.
- Begin injecting as soon as the first reference peak is nearing completion. Inject a sample as soon as the previous injection sample peak (not air peak) begins. Inject 7 samples.
- When all timed events are done, turn valve back to EA then change the septum. If not using anymore, turn GC to 40°C.
- For 10% gas samples, you should inject around 0.2 mL
- Atmospheric concentrations of gas should be injected around 5 mL and need a cooloer GC temperature.
- Peak area should be around 50 Vs
- If no peaks occur after injection then the syringe may be clogged.
- If peaks are occurring faster than normal, there may be water in the GC.
- If peaks are very broad, try and inject the sample quicker.
Fessenden J, Cook C, Lott M, Ehleringer J. Rapid 18O analysis of small water and CO2 samples using a continuous-flow isotope ratio mass spectrometer. Rapid Commun. Mass Spectrom. 2002; 16: 1-4