EPA Method 18
VOC by GC - Flexible Bag Sampling
Method Overview
- • A lung system is used to pull a DRY sample into a flexible
bag.
- • Moisture should be removed before the bag, and will usually
require separate analysis.
- • Typical bag materials include Tedlar, Teflon, and Mylar
coated polyethylene.
- • Analysis is by gas chromatography. (GC)
Target Compounds
- • Compounds with vapor pressure above 1 mm Hg at room temperature.
- • Compounds that are not reactive.
- • Compounds that do not adhere to the bag surface (e.g. not
good for alcohols, DMF, amines, etc.)
Method Summary
- • Extract sample from the stack at a constant rate. Set rate
so that the bag (e.g. 10L) is greater than 50% full, but no more
than 70% full at the end of the run.
- • Use a lung system so that the sample does not contact the
pump.
- • Avoid connections made with Tygon tubing or surgical tubing
as these tubes may act as adsorbing surfaces to cause lower than
real results. For the same reason, avoid bags with plastic fittings
or neoprene O-rings.
- • After the bags are analyzed, one from each location is
selected for spiking. ALL compounds of interest are spiked at 40%-60%
of the average concentration. The amount recovered is used to calculate
Recovery factor "R".
- • The spiked bag is re-analyzed AFTER an interval equal to
the duration that the original sample was held in that bag before
analysis. E.g., typically 2 hours for on-site projects, and at least
24 hours for samples shipped overnight to the lab.
- • The spiked bag must meet 70% < R < 130%; all other runs
are then corrected with the recovery factor.
M18 2/2000 Updates include important changes in addition to the rule
that the spike must be held in bag for the same duration that sample
was in bag before analysis. Study updated method before sampling!!
- • Study the method!
Quality Assurance
- • Always leak check bags before use (fill and leave under
pressure overnight, check for deflation).
- • Use new bags, or check for contamination before use.
- • Determine a recovery efficiency (factor) "per compound,
per source".
- • The Method requires triplicate injections agreeing ±5%
of their mean.
- • The Agency often provides Method 18 audits.
Shipping Tips
- • Bags are time critical - Please notify us well in advance
so that we may schedule & setup an instrument for availability
when your bags arrive.
- • Do not ship bags on ice (it's been done before!!).
- • Never ship a completely full bag: bags should be inflated
no more than 70% before air transport.
- • Unless from an inlet location, most bags are typically
not shipped as hazardous goods.
- • Knockout condensates should be zero head-spaced before
shipping. Add water if needed to remove headspace in the VOA.
- • Consider 20 mL VOAs as knockouts for dryer sources.
- • Notify us ahead of time!
- • Bags arriving before 12:00 with prior notice will be analyzed
on date of arrival.
- • Bags arriving after 12:00 will be analyzed within 24 hours.
- • Please co-ordinate Saturday deliveries: usually we request
that you ship to a specific local FEDEX station.
Enthalpy Experience
Enthalpy's staff has performed Method 18 bag analyses for 12 years
- our combined M18 bag analysis time is well over 30 years. Enthalpy
operates an autosampler of our own design built specifically for bag
analyses, and is one of the few labs maintaining dedicated equipment
for correctly performing bag analyses. Please consult with us before
agreeing to a M18 bag testing protocol. Our experience may help you
avoid negotiating a test plan that is not optimized for success.
