EPA Method 18 - Adsorption Tubes




Method Overview

  • 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)

Target Compounds

  • 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.)

Performance Tips

  • Sample is withdrawn from the source at a constant rate and through a series of adsorption tubes.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.

Shipping

  • 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

Applications

  • 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.

Accreditation

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

White Papers