Sample collection for dissolved gas in GW

[jearles]

Just wondered if anyone had suggestions for alternative sample collection of groundwater to do dissolved gas analysis? I've only used passive diffusive samplers in the past so I'm not very familiar with options for this type of sampling. Submerging serum bottles in groundwater then putting in stoppers is problematic for some of our field techs physically (e.g. arthritic hands), especially since this is a long term sampling campaign with many wells to sample. So I'm trying to find alternative methods to make it easier.

I saw that microseeps has a VOA type vial with special septa for dissolved gas analysis which would be perfect, but hadn't found these available to purchase. Anyone know where to buy something like this or have other suggestions?

Thanks!
Jennifer

[Bigbear]

VOA septa (PTFE faced) vials should work. I use them for EPA 524 and can see vinyl chloride as well as the chlorofluoro methanes I callibrate for.

[Steve Reimer]

This topic came up a while ago in a discussion about vinyl chloride in groundwater. A research hydrologist at a government agency offered some advice. I’ll paraphrase here for brevitiy:

As a gas, VC losses can be substantial if the container is not gas tight. Likewise sorption to stoppers can be a problem, so teflon coated stoppers are recommended. …flat septa (may be) unreliable for long-term storage of water samples for VOC analysis. Use teflon coated stoppers (Wheaton manufactures one) and crimp seals. Likewise, it takes experience with sealing VOA vials and personally analyzing those samples to recognize the vulnerability of such containers to poor container closure techniques, a particular concern for contractor collected samples. In other words, it is very easy to get an analytical result with little appreciation for the quality of that analytical result.

…volatile samples are best collected using modifications of techniques developed for collection of environmental oxygen analyses. The collection outlet (outflow tubing for pumped samples) should be placed in the bottom of the VOC container and the sample allowed to overflow for 3 times the container volume. Then the tubing is carefully removed, leaving an undisturbed convex meniscus of water. Preservative is added to the top of the meniscus with a dropper.The sample is then closed carefully.For VOC samples the volume of sample required to fill the container once is always compromised by off-gassing.

While our discussion was on the topic of preservation his point was that the way samples are collected can have a large affect on the results. It would be best if you consult with someone experienced in collecting samples for dissolved gasses before you start.

[twranger]

I'm not familiar with sampling procedures, but I think the septa Microseeps is referring to comes from the RSK-175 SOP, which recommends butyl rubber teflon faced septa, instead of silicone. We are using the PADEP method, which uses normal VOA vials and sampling for methane/ethane.
Also, as I understand it, dissolved oxygen should be measured in the field as it is near impossible to sample correctly and maintain field levels.
The USGS has alot of experience with sampling, they may be able to help:
http://water.usgs.gov/lab/dissolved-gas/sampling/

[jearles]

Thanks for all the replies so far!

twranger we do a separate DO measurement in the field, so that is taken care of. We've been using both the RSK175 and USGS information for guidelines, the method I use is based on the RSK175.

I'm wondering if a headspace vial like this could work, since they are available with PTFE/Red Chlorobutyl caps?
http://www.restek.com/catalog/view/9005 http://www.restek.com/catalog/view/9006

Or this?
http://www.sigmaaldrich.com/catalog/pro ... &region=US

If it helps to know, we look at H2, N2, O2, CH4, CO2, N2O, using a TCD. Samples are analyzed the same week they're collected.
Thanks again for the help!