As part of my doctoral research, I will need to do some field work, and I need a portable GC unit for detecting methane at the ppm (parts per million) level. My study area is Inuvik, NWT, Canada, far above the Arctic circle and far removed from suppliers of scientific equipment. I can't count on the availability of a lab unit at the local research laboratory in Inuvik. I am aware that portable GC units exist, but I would like to know from someone with experience on the subject about how these units compare to lab-based models. Basically, are they worth the money? What kind of power supply do they need? I am also anticipating a problem with standards. Any information would be appreciated.
By John Hinshaw on Friday, November 19, 1999 - 02:57 pm:
There are a number of commercially available units you might investigate. HNu, PerkinElmer's Photovac line, and Agilent Technologies portable instruments come to mind immediately.
Some of these units are field-hardened for use in explosive environments, and most will run for 4-8 hours on a single gas / battery charge. Lab instruments run continuously, of course, but you must collect samples in the field and bring them back for analysis. "Grab" bags or evacuated cannsters are commonly used for this purpose.
Are you interested in only methane at ppm levels, in a number of the low carbon-number hydrocarbons commonly found in the atmosphere, or in a total hydrocarbon analysis?
Calibration requires one or more calibrated gas standards, which are available from most specialty gas companies.
Some of these units have built-in software that will record multiple analyses on-the-fly, too, or they hook up to a laptop.
By mmarsch on Monday, November 22, 1999 - 01:22 pm:
I am currently looking only at methane, not at longer-chain hydrocarbons. In the lab, I use the GC-FID for analysis of methane and methanol. The methanogenic bacteria with which I work produce only CH4, not C2H6 or C3H8 etc. I am aware that I could also analyze for carbon dioxide if I had a TCD, but I get data for CO2 from doing separate dissolved inorganic carbon (DIC) extractions for stable isotope analysis. TCD is not particularly sensitive to methane. I am basically wondering if a portable GC unit can produce results comparable in accuracy and reproducibility to lab units, making the investment worthwhile instead of taking gas samples back to the lab long after they were collected.
How would you go about getting methane standards to a distant place like Inuvik? I think it is illegal to ship cylinders of explosive gas by air.
By John Hinshaw on Monday, November 22, 1999 - 05:07 pm:
Methane standards are not explosive because they are already diluted to a working level such as 100 ppm in nitrogen. They are under a few hundred pounds of pressure, which may cause a problem on commercial airlines. You should ask the airline's shipping dept. about how to send such items.
By Bill Jenko on Wednesday, November 24, 1999 - 07:57 pm:
Small, disposable cylinders of 100 ppm C1 in N2 at low pressures are available, and may be shipped by air. I am not sure that they can be shipped in passenger aircraft, but via cargo aircraft is certainly not a problem. Any reputable gas supplier (Matheson, Scott, Gaspro, Air Liquide, MGI, etc.) will be happy to supply these.
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