EPA Method 18 - Flexible Bag Sampling

Method Overview

  • A lung system is used to pull a (dry) sample into a flexible (non-reactive - Tedlar) bag
  • Moisture should be removed prior to the bag (e.g. in midget impingers)
    • Any condensate in impingers would require a separate analysis
  • Typical bag materials inlcude:
    • Tedlar
    • Teflon
    • Mylar coated polyethylene
  • Sample degradation issues are dealt with using a spike/recovery procedure
  • Analysis is by gas chromatography (GC)

Target Compounds

  • EPA Method 18 is suitable for most organic compounds
  • Unsuitable compounds are those that:
    • Are polymeric (high molecular weight);
    • Can polymerize before analysis; and/or
    • Have very low vapor pressures at stack or instrument conditions.
    • Reactive compounds
    • Compounds that can adhere to the bag surfaces (e.g. amines, some alcohols, etc.)
  • Generally for compounds with a vapor pressure above 1 mm Hg at ambient conditions
  • The lower range of this procedure is determined by the sampling system; adsorbents may be used to concentrate the sample, thus lowering the limit of detection below the 1 ppm level typically achievable with direct interface sampling.
  • The upper limit is governed by teh GC detector saturation or column overloading. The upper range can be extended by dilution interface sampling.

Performance Tips

  • Sample is withdrawn from the source at a constant rate and fed into the flexible bag. Sample flow rates should be set such that the bag is:
    • A minimum of about 50% full at the end of the test run; and
    • No more than about 70% full at the end of the test run.
    • Full bags can explode during shipping (cargo area pressure drops during air freight).
  • All heated elements should be set to ~250oF;
  • Use of a lung sampling system will assure that the sample does not contact pump head surfaces
  • Connections made using Tygon or Surgical Rubber tubing should be avoided
    • These materials can absorb analytes, lowering the measured results
    • Bags with neoprene O-rings should also be avoided - for the same reason
  • A recovery study must be performed following the analysis;
    • Performed by spiking one out of three samples with between 40%-60% of each target analyte;
    • This spiked sample must sit the same amount of time as occurred between sampling and analyses
    • Analysis results are typically corrected for this recovery study results
    • The spike recovery must be between 70% < R < 130% for the testing to be considered valid.


  • Flexible bag analyses are time-critical
    • Advance notification will allow Clean Air to have an instrument calibrated for each compound and ready to go once samples arrive
    • Certain reactive compounds can require overnight analysis (e.g. ethylene oxide can lose about 1% per hour in a bag)
  • Do not ship samples on ice! (It's been done) This can cause condensation inside the bag.
  • Do not ship a full bag. Bags should hold no more than about 70% of their capacity prior to air shipment
  • Most samples do not require hazardous shipping (unless from a fuel source, etc.)
  • Knock-out condensates should be zero headspace prior to shipping. Completely fill the sample container with organic-free water
    • You might consider 20 mL vials for many locations
  • Notify us ahead of time
    • Bags arriving on an overnight basis (10:30 am) can be analyzed on a same-day basis
    • Bags arriving in the afternoon may have to wait another day for analysis
    • We do have an autosampler capable of handling overnight analysis of flexible bag samples
    • Please coordinate Saturday deliveries with us AHEAD OF TIME!


  • A main use is coupled with EPA Method 25A Sampling.
    • Methane and Ethane are the analytes here for subtraction from the THC Emissions.
  • Emissions of generally low concentrations (0-100 ppm range) of many analytes.
  • The presence of water (condensation) can negate the use of bag sampling.
  • Does not work well for polar compounds (especially alcohols).


  • Yes we are, please contact us if you are interested!
    • New Jersey
    • Texas
    • Louisiana

White Papers