EPA Method 18 - Direct / Dilution Interface




Method Overview

  • The sample is withdrawn directly from the source through heated line to the GC
  • Analysis is semi-continuous with a snapshot every 5-20 minutes
  • Method 18 requires a minimum of five injection per test run
  • Sample degradation issues are minimized by direct injection into the GC
  • Direct injection is an excellent tool for tracking batch processes (unless the process changes can occur faster than the snapshot injection times)

Target Compounds

  • EPA Method 18 is suitable for most organic compounds
  • Unsuitable compounds are those that:
    • Are polymeric (high molecular weight);
    • Can polymerize before analysis; and/or
    • Have very low vapor pressures at stack or instrument 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 teh GC detector saturation or column overloading. The upper range can be extended by dilution interface sampling.

Performance Tips

  • Sample is withdrawn from the source and delivered continuously to the GC (similar to EPA Method 25A). A preferred sampling rate would yield a response time of less than one minute.
  • All heated elements should be set to ~250oF;
  • A slip-stream of the sample is drawn through a gas sampling valve on the instrument;
    • An injection is performed by rotating the valve;
    • Each injection will typically require 5-20 minutes to perform the analysis.
  • A recovery study must be performed following sampling (similar to sample system bias);
    • Performed by introducing sample to the probe tip and sampling similar to stack gases;
    • Recovery is acceptable if bias is within +10% of the at-instrument value.
    • Analysis results are typically not corrected for this recovery study results
  • Short lengths of unheated lines can cause sample loss through condensation (e.g. 4" of teflon line). All sample delivery lines should be heated.

Shipping

  • No shipping issues - samples analyzed on-site

Applications

  • Fuel gas analyses (Btu Content);
  • Emissions of almost any VOC from process vents;
  • Method can (and probably should) be coupled directly with THC (EPA Method 25A) Continuous Measurements.
  • Destruction and Removal Efficiency of individual Analytes
      Simultaneously Measure at Inlet and Outlet of a control device.

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