Method 18 - Tubes

non-Isokinetic sampling
Moisture should be removed prior to the adsorption tube (e.g. using midget impingers)
- Any condesnsate in impingers would require a seperate analysis
Typical adsorbent materials inlcude:
- CSC Charcoal
- Tenax
- XAD Resin
Sample degradation issues are dealt with using a duplicate sampling train coupled with a spike/recovery procedure
Analysis is by gas chromatography (GC)

EPA Method 18 is suitable for most organic compounds
Unsuitable compounds are those that:
Generally for compounds with a vapor pressure above 1 mm Hg at stack 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 the adsorption tube saturation limit or overloading. The upper limit may be as low as 50 ppmv for certain compounds (using adsorption tubes).
Not generally suitable for highly volatile organics (methane, ethane, etc.)

Sample is withdrawn from the source at a constant rate and through a series of adsorption tubes.
Duplicate sampling trains with one pre-spiked for each analyte is required
- A spike recovery must be between 70% < R < 130% for the test run to be considered valid
- Multiple adsorption tubes will have to be analyzed to show sampling train collection efficiency
Sample flow rate is determined by analyte concentration and specific adsorption media
Moisture should be removed by a water knock-out
- The method requires that the moisture catch be analyzed for each analyte
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
Adsorption tubes are recovered and analytes are desorped using a solvent
Sorbent selection will depend on the anlaytes (example: naphthalene will not desorb from CSC charcoal)
The solvent is then analyzed
A recovery study must be performed;
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.

Sorbent tubes should be kept chilled after removal from the sampling train
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
- Samples arriving on an overnight basis (10:30 am) can be analyzed on a same-day basis
- Samples arriving in the afternoon may have to wait another day for analysis
- Please coordinate Saturday deliveries with us AHEAD OF TIME!
Always includee Chain of Custody documentation with the samples

Especially useful where very low Method Detection Limits are Required
- The adsorption tube basically concentrates the analyte prior to analysis.
Sorbent Choice may be limited depending on the analyte
- Example: Naphthalene does not work well on activated charcoal.
- Recovery can be less than 70% from charcoal for this analyte;
- Recovery will average 100% from XAD tubes for this analyte.
- Contact us for any questions about sorbent choice.