Acetylene, N2O and GC-ECD

[aqualab]

Hi There,
We are trying to measure N2O emissions from 50ml acetylene gassed soil slurries in closed 250ml containers using a GC-ECD. We are currently injecting only 50 micron of the containers headspace but are getting a lot of interference from water vapour, and the acetylene seems to make things worse. At the moment we are burning off every 9 injections or so. Does anyone have any setup sugestions that could possibly reduce the amount of burnoffs? At the moment the run temp is 50 degrees and burn off is at 150 degrees, with a splitless setup and helium carrier gas. We also have access to a variety of columns, split ratios and carrier gases, but none of us know much about GC-ECD and are not sure where to start tweaking first. Would any of these variables have more of an effect than others?

[MikeD]

My application is not headspace and I don't know if this helps, but here are the GC conditions I use for GC-ECD of nitrous oxide.

Column - 60 m x 0.32 mm GasPro (J&W/Agilent); He - 21 psi; temp - 150C isothermal for 8 mins then heat to 250C for 2 mins; ECD temp - 340C. N2O elutes in about 7 mins. Water and hydrocarbons do not interfere.The ECD response to N20 is temperature sensitive, at least it is for the PerkinElmer ECD. The response increases 4x going from 250C to 340C.

I assume you are injecting 50 microlitres splitless. The PerkinElmer ECD has a dynamic range of 2-300 nanograms N2O. With your injection volume that translates to 20 - 3000 ppm, the lower figure being the quantitation limit. Do you have enough sensitivity ?

[Ron]

You probably need to have a valve in your system to vent the water either to another column or to waste before it enters the ECD. You should be able to easily measure sub-ppm concentrations of nitrous oxide, the analysis of nitrous oxide in air was one of the first ECD applications.

[MikeD]

Ron,

I agree about the ability of the ECD to measure sub-ppm N2O if you use 1-5 ml samples sizes that are the norm for soil gas. I couldn't do this on my system with 0.05 ml. If someone has an ECD 20-50X more sensitive than a Ni63 PerkinElmer then OK they can do it (just). Early measurements of 0.3 ppm ambient N2O in air used a preconcentration step either on cryo traps or Mol Sieve 5A. I also preconcentrate on Mol Sieve 5A in my application, although that's partly because I use a split injection.