EPA Method 18
VOC by GC - Sorbent Tubes
Method Overview
- • Non-isokinetic.
- • Pollutants are adsorbed onto media contained in glass tubes,
then recovered in the lab.
- • Analysis by gas chromatography. (GC)
- • The Method requires recovery efficiency determination.
Target Compounds
- • Organic compounds with vapor pressure between .1 mm and
300 mm at room temperature - e.g., alkanes C5 to C14, aromatics,
chlorinated alkanes, alcohols to C18, ketones.
- • NOT suitable for: highly volatile organics, reactive organics,
and inorganics.
Method Summary
- • Sample is pulled from the source at a constant rate and
through a series of glass tubes containing adsorbent media. Many
sorbent media types are available, and media choice depends on the
compounds of interest. The flow rate is determined by the expected
concentration and the capacity of the media.
- • Moisture is removed by a water knock-out.
- • Immersing the media in a solvent that has a high affinity
for the target compounds strips, or desorbs, those compounds from
the media.
- • Solvent is then analyzed by GC, usually with flame ionization
detector (FID).
- • The method requires that the moisture removing knock-out
impinger is analyzed.
- • A recovery factor must be determined 'per compound, per
media, per source'. Check with the regulator for project specific
interpretation of this requirement.
- • The recovery factor is determined by 'spiking' all compounds
of interest onto a second train. The spiked train is run 'co-located'
(alongside and simultaneously) with the sample train. Both trains
are analyzed and the amount of the spike recovered (R) is calculated.
The average R for three runs is used.
- • Recovery Factor must be >70% and ‹130%.
Quality Assurance
- • Reagent blank of desorption solvent.
- • Field blank.
- • Recovery Efficiency runs (typically 3 per source / location).
- • Lab Control Spikes - (LCS) - (typically 2 per set of spiked
tubes). Enthalpy does not charge for LCS analysis.
Shipping Tips
- • Tubes must be kept chilled after removal from the sampling
train.
- • Knock-out condensates must be zero head-spaced before shipping.
Add water to remove any headspace in the VOA. Consider 20 mL VOAs
for dryer sources.
- • Samples rarely require hazardous shipping.
- • Never send for Saturday delivery without notifying us,
lest the tubes warm up over a week-end.
- • Consider shipping tubes inside Nalgene bottles to prevent
breakage and getting the labels wet.
- • Always include a complete Chain of Custody.
Enthalpy Experience
Enthalpy has specialized in EPA Method 18 tube projects from our
inception, and performed thousands of tube analyses. Please consult
with us before agreeing to a test protocol. Our years of experience
may help you avoid negotiating a test plan with a less than optimum
chance for success. Enthalpy's founders were stack testers and have
performed hundreds of adsorbent tube field tests. Our principals have
delivered papers on the subject to AWMA, the Engineering Foundation,
at training seminars across the country, and commented on Method 18
drafts at the request of the Agency.
