CO₂ Injections

Introduction

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.

Procedure

1. 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)
2. Within the acquisition window, switch the gas to N2 and have dilution OFF.
3. With dilution OFF, watch the N2 (mass 28 and 29) levels. They should be at or below 10 mV.
4. 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.
5. 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.
6. 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.
7. When all timed events are done, turn valve back to EA then change the septum. If not using anymore, turn GC to 40°C.

Injection Volumes

1. For 10% gas samples, you should inject around 0.2 mL
2. Atmospheric concentrations of gas should be injected around 5 mL and need a cooloer GC temperature.
3. Peak area should be around 50 Vs

Troubleshooting

1. If no peaks occur after injection then the syringe may be clogged. 
2. If peaks are occurring faster than normal, there may be water in the GC. 
3. If peaks are very broad, try and inject the sample quicker.

Suggested Reading

• Fessenden J, Cook C, Lott M, Ehleringer J. (2002) Rapid 18O analysis of small water and CO2 samples using a continuous-flow isotope ratio mass spectrometer. Rapid Communications in Mass Spectrometry 16, 13. doi: 10.1002/rcm.711.

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