EPA Method 18 - Adsorption 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
- 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- Sample is withdrawn from the source at a constant rate and through a series of adsorption tubes.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
Clean Air does have temperature loggers available for continuously recording sample temperatures
- 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.
- Yes we are, please contact us if you are interested!
- New Jersey