In the past I have used a small, field portable, slide-hammer-driven soil gas sampler (a hollow metal tube) with a soil gas sampling point that slides in the end.  The point is connected to tubing, which is connected to a small pump (even a fish tank pump could work).  This sucks the gas from the ground through the tubing.  After you have driven the point to the desired depth, you must then evacuate the gas in the tubing to make sure there is no air contamination.  After that you would connect the outflow from the pump to the tedlar bag for sampling (probably 1 L is fine).  I have also used small evacuated canisters (< 1L volume) instead of bags as well.

Hi Mark, having used the above setup I can also vouch for its effectiveness.  However, I had difficulties with the tedlar bags. Specifically, if they are not analyzed within 48 hours they leak and or I saw major fractionation. I ended up using evacuated glass flasks.  Another method that Simon had success with was the use of exertainers. Small purged vials. I use these now as they are small, strong and can hold a sample for a long time without issues. Cheers Clint

Thanks Cindy and Clint.  Cindy -  I am keen to know if you have suggestions on where to source the slide-hammer-driven soil gas sampler.  Clint I really like the idea of exertainers, but checked them out and they look too small.  I need about 300mL for the Picarro 13C analyser.

Hi Mark, Travis Horton of UoC has used them for 13C of CO2 for some time. He also runs a Piccaro CRD.  Cheers Clint