Robin,

This may be the information, supplied by Geotechnical Instruments, that you are looking for: 

"Methane is measured on-site using an infra-red gas analyser.  This works by measuring the concentration of gases that absorb radiation in the infra-red region of the electromagnetic spectrum.  This is achieved by careful selection of a unique infra-red wavelength for measuring the gas.  In practice, because the infra-red spectra of different gases, particularly organic hydrocarbon gases and vapours, often overlap, the selection of a unique measuring wavelength has proven to be difficult, and it may be true that the specificity of methane can be reduced by interference.

With this equipment methane is read by filtering to an infra-red absorption frequency of 3.41 um (nominal), the frequency specific to hydrocarbon bonds.  Analysers are calibrated using certified mixtures and will give a correct reading providing there are no hydrocarbon gases present within the sample (e.g. ethane, propane, butane).  If there are other hydrocarbon gases present, the methane reading will be higher than the actual methane concentration being monitored.  The extent to which the methane reading is affected depends upon the concentration of the methane in the sample and the concentration of the other hydrocarbons".  

I agree with Mike, in terms of vapour sampling it is preferable to collect samples directly onto the extraction media of choice and desorb the material for lab analysis.  The EA publication, Investigation of the Composition and Emissions of Trace Components in Landfill Gas, suggests that sub-sampling from tedlar bags should be used with caution because of the stability of a number of target compounds.  To assure the quality of the data the recommendation is to use mixed bed thermal desorption tubes with subsequent analysis by GCMS.  The technique is particularly suited to coupling with gas chromatography, which allows the minimum of sample preparation.  The analytes are pre-concentrated during sampling and consequently this method does not suffer from the problems associated with the grab sample techniques, such as doubts about sample authenticity during the period between sample collection and analysis.  

Regards,
 
Geraint Williams
Environmental Scientist
 
Alcontrol Laboratories,
Units 7 & 8, Hawarden Business Park
Manor Lane, Hawarden
Deeside, CH5 3US

Direct Dial:   +44 (0)1244 528 749
Fax:      +44 (0)1244 528 701
Mob:     +44 (0)7710 613 678


-----Original Message-----
From: Contaminated Land Management Discussion List
[mailto:[log in to unmask]]On Behalf Of Robin
MacKenzie
Sent: 29 May 2009 15:24
To: [log in to unmask]
Subject: Gas measurements


Sorry not in office next week but will get Tom to email the Cross Gas lit. Makes you wonder though how much percentage of gas is actually CH4 when you are measuring putrescent waste landfills. Maybe a comment from the experts on this one?

Have a nice weekend.
Robin Mackenzie
Contaminated Land Co-ordinator
Perth and Kinross Council

Tel 01738 476443
Mob: 07810057566

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