EPA Method 18
Direct / Dilution Interface (Onsite Technique)
Method Overview
- • The sample is drawn directly through heated line from stack
to the GC.
- • Analysis is semi-continuous with a "snapshot" every 5-20
minutes.
- • Method 18 requires 5 injections per run.
- • No sample degradation issues.
- • M18 QA is completed before demobilizing.
- • Direct interface is excellent for tracking batch processes
unless events happen faster than GC cycle time (5-20 min).
Target Compounds
- • M18 direct interface is suitable for most organic compounds
and some inorganic compounds (e.g., SF6).
- • Compounds unsuitable for direct interface are those that
exhibit very low vapor pressure, are reactive, or have a high affinity
for the components in the sample train (for example: glycerine).
- • No upper concentration limits apply when using a dilution
system (Enthalpy can supply dilution systems).
- • Lower limit is defined by the response of specific compounds,
and is typically less than 1 ppmv.
Method Summary
- • Draw sample from the stack at a constant rate and transport
to the GC (similar to EPA M25A). Preferred sampling rate provides
a response time of ~ 1 minute in the sampling system.
- • All heated elements should be set to 250°F.
- • A slip stream of the sample is drawn through a gas sampling
valve on the GC; an injection made by rotating the valve. Each injection
requires 5 to 20 minutes to complete analysis.
- • The recovery study is performed before sampling by introducing
the mid level calibration gas to the probe tip and drawing it through
the system just like sample gas. Recovery is acceptable if the system
check is +/-10% of the direct "to the instrument value". Sample
results are NOT corrected for recovery.
- • Only 4" of unheated Teflon or 1" of unheated stainless
steel can cause condensation in the sample system.
- • The system is leak checked prior to the recovery study.
Quality Assurance
- • Calibrate on-site with triplicate injections.
- • Perform recovery study for at least one target compound.
- • Perform post-test calibration.
- • Agency provides audits.
Pricing Information
- • Rates are situation dependent. However, $2,000-$4,000 for
prep and mobilization (depending on complexity and distance from
Raleigh) plus $1,200 per onsite day is typical.
- • Rates for onsite projects in the southwest and south central
regions of the U.S.A are minimized by Enthalpy's complete set of
field equipment that mobilizes from Houston, Texas.
Shipping Tips
- • No shipping issues - all samples are analyzed on-site.
Enthalpy Experience
Enthalpy's analysts have performed Method 18 by direct interface
for 13 years, with over 30 collective years of experience. Enthalpy
routinely performs on-site analysis on a subcontract basis for or
with stack test companies. Enthalpy will work in your trailer or mobile
lab. We provide sampling pumps, GC, operator, and calibration gases.
Our clients supply the sampling probe, heated line, fuel gases, and
a suitable location for the GC.
