Method 18 - Direct

The sample is withdrawn directly from the source through heated line to the GC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)

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.

Sample is withdrawn from the source and delivered continuously to the GC (similar to EPA Method 25A).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.

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