EPA Method 18 - Flexible Bag Sampling
- 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:
- Mylar coated polyethylene
- Sample degradation issues are dealt with using a spike/recovery procedure
- Analysis is by gas chromatography (GC)
- 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.
- Sample is withdrawn from the source at a constant rate and fed into the flexible bag.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