The International Information Center for Geotechnical Engineers

Landfill Gas Monitoring Systems - Sampling Techniques

 3.3 Sampling Techniques

 A lot of techniques can be used for soil-gas sampling.The following list brings the main techniques used in-situ:

  • Reactive tubes
  • Device using photo-ionization (photo ionization detector - PID)
  • Measuring device detecting infrared (IR)
  • Device using flame ionization (flame ionization detector - FID) 
  • Photo-acoustic device
  • Device using gas chromatography which provides a precise concentration value for VOC (volatile organic compounds) / COHV (highly volatile organic compounds). Among the basic detectors used, FID is usually used for BTEX (benzene, toluene, ethylbenzene, and xylenes, which are main compounds of VOC) and ECD (electron capture detector) for COHV.

After sampling from the field, the gas sample is usually sent to the labrotary, and analyzed using GC-MS technique (Figure 10). GC (Gas Chromatography) is used to seperate gas mixtures into individual components using a temperature-controlled capillary column. MS (Mass Spectrometry) is used to identify the various components according to their mass spectra.


  Figure 10. GC-MS Technology (Evans Analytical Group)


In the following part, three sampling methods are introduced. The first two are active sampling using pumps, and the last one is a passive sampling (sample the gas using pressure difference).


 3.3.1 Soil-gas sampling using sample bags

 Sample bags are usually used for permanent soil-gas sampling (CO2, NOx...) or ambient air sampling. Nowadays, different sorts of sample bags are available and the appropriate sampling bag should be selected depending on their properties. This method can be applied with both types of installation wells (temporary and permanent).

A sampler bag requires a pump, which is connected to the soil gas well, to pull the gas from the soil, and an inert hose (can't absorb or react with the gas components) to link the sampler bag to the pump.  The sample bag size can be adjusted depending on the volume of gas necessary for the analysis (from some milliliters to several dozen liters). But The sample bag should not be filled more than 2/3 full. Then, depending on soil characteristics, soil-gas installation designs and the targeted quantification limits, sampling duration and pumping flow are determined.

After sampling, the sample bag should be stored out of sunlight, at room temperature. In the lab, gas is collected by pumping from the sample bag to an adsorbent tube, and then analyzed by GC-MS.


 3.3.2 Soil-gas sampling using sorbent tubes

 Sorbent tube is the most used sampling technique for soil-gas sampling. It can also be used for ambient air sampling. The tube is typically made of glass and contain various types of solid adsorbent materials (sorbents). The sorbents can trap and retain the compound(s) of interest in the presence of other compounds.

This method also use a pump to pull the gas from the well and let the gas go through the sorbent tube. The tube is linked to the pump by inert hose.


Figure 9

Figure 11. Sample bags (INERIS)


Sampling duration, pump flow, pressure, temperature should be well-mastered during the sampling event for better calculation and interpretation. Temperature and pressure may influence the volume of gas sampled. If these two parameters are monitored, recalculation can be considered to provide results under normal conditions. (CityChlor, 2013)

Once the sampling period ends, tubes should be storage in cold (< 4°C) and dark environment until their analysis. The analysis is carried out in a specific laboratory. First, trapped compounds are extracted by thermal desorption or chemical desorption and then quantified by GC-MS. Various analytical methods have been established for sorbent tubes analysis: EPA TO-17, ASTM D6196, ISO 16017, ISO 16000-6 or NIOSH 2549. Depending on the mass of the compound extracted from the sorbent tubes, the sampling flow and the sampling duration, concentration can be calculated.


Figure 10

Figure 12. Soil-gas sampling with sorbent tubes (INERIS)


3.3.3 Soil-gas sampling using Summa-Canister®

Summa-Canister® is a stainless steel container which the internal surfaces have been treated (summa process) to avoid adsorption of compounds. The use of Summa-Canister® is particularly helpful when extreme concentrations are met (both high and low concentrations). This method is different from the above two because it doesn't need a pump to sample the gas, so it's a passive sampling method. It can be used in ambient air sampling as well as soil gas.

The canister is evacuated just before it is sent to the field. The valve is opened at the beginning of the sampling event and the gas enters into the canister spontaneously (because of the difference of air pressure). This valve can be equipped with a flow controller for filling in a targeted flow rate. Filling of the canisters is controlled by the pressure gauge fitted on them.When sampling ends, the valve is closed and the whole system is sent to the analytical laboratory. Storage and transport should be carried out at room temperature. The canister is bulky and heavy, so it's difficult to transport and store it.

When returned to the laboratory, the sample should be analyzed as soon as possible. Gas is collected by pumping the gas from the canister to an adsorbent tube, then analyzed usually by GC-MS.


Figure 11

Figure 13. Summa-Canister (INERIS)


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