Method 18 - Bags

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)

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Sample is withdrawn from the source at a constant rate and fed into the flexible bag.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).