1116 STACK SAMPLING
chromatographic column where the TGNMOs are separated
from CO 2 and methane. Using an unusual backflush proce-
dure, the TGNMOs are then oxidized to CO 2 , catalytically
reduced to methane, and analyzed using a flame ionization
detector. By measuring the organics in this way, the differ-
ence in flame ionization response factors for various organic
compounds that may be present is eliminated.
There are some limitations of this method, which the
analyst should be familiar with.1) Organic particulate matter will interfere with the
analysis, giving higher values than actual. This
can be eliminated by using a heated filter.
2) The minimum detection limit is 50 ppm as
carbon.
3) When both water and CO 2 are present in the stack
gas there can be a high bias in the concentration of
the sample. This is due to the inability of the chro-
matographic column to separate the carbon diox-
ide from the hydrocarbons when large amounts
of CO 2 and water are present in the sample gas.
When the product of the percent water times
the percent CO 2 is greater than 100, this bias is
considered to be significant. In such cases, other
methods should be used.An emission sample is drawn at a constant rate through a
heated filter and then through a condensate trap cooled with
dry ice into an evacuated sample tank. The probe is heated to
maintain an exit gas temperature of at least 129ºC (266ºF).
The filter is heated in a chamber capable of maintaining
a gas temperature of 121 3ºC (250 5ºF). The sample
tank must be leak checked and cleaned by the procedures
described in the method. The pressure of the sample tank
is measured before and after sampling, and this information
is used to determine the amount of sample collected. After
sampling the sample tank valve is closed and the condensate
trap is capped.
The condensate trap will contain water, CO 2 , and con-
densed organic compounds. The CO2 will interfere with
the analysis of the organics in the trap. CO2 is removed by
purging the trap with clean air into the original sampling
tank. The method describes this procedure in more detail
and includes a figure showing the recommended equipment.
After the purge is complete, the sample tank is pressurized
to 1060 mm Hg absolute. The condensate tube is then con-
nected to an oxidation catalyst and heated while purging with
air or oxygen. The water collected with the sample and that
produced by the oxidation of the hydrocarbons is removed
with a cold trap. The CO 2 produced from the oxidation of the
condensed organic compounds is collected in an intermedi-
ate collection vessel.
The analysis of the gas in the sample collection tank and the
intermediate collection vessel both use gas chromatographic
separation. The gas chromatograph column and operating
conditions are chosen to separate CO, CH 4 , and CO 2. These
compounds are first oxidized then reduced to methane that is
quantified by the flame ionization detector. The non-methaneorganics are retained on the column. A valve then reverses
the flow of carrier gas through the column, back flushing the
organics off the column through the oxidation then reduction
catalyst before going to the detector. The TGNMO concen-
tration is the sum of the non-methane organics and CO 2 from
the intermediate sampling vessel.
The Method requires extensive quality assurance and
quality control measures to insure that valid data are pro-
duced. The analysis of two audits gases provided by the
Emissions, Monitoring and Analysis Division at Research
Triangle Park, NC is required in the Method. This method
should be performed only by personnel familiar with the
sampling method and the gas chromatography necessary for
the analysis. There have been recent changes in this Method;
therefore it should be read carefully by all persons involved
in its use.Test Method 25ATest Method 25A uses a Flame Ionization Analyzer (FIA)
that has a flame ionization detector to directly measure that
total gaseous organic compounds from a stationary source.
A gas sample is extracted from the center of the stack. The
sample is filtered if necessary to remove particulates which
could give high readings or clog the detector. The concentra-
tion is expressed in terms of propane or the concentration as
carbon is calculated.
This method is much simpler to perform than Method- However, there are two major limitations that must be
understood.
1) The FIA will not separate or identify the organic
compounds present.
2) The response of the detector is different for each
compound.Method 25A is the method of choice for the following situ-
ations.1) When only one compound is present.
2) When the organic compounds present consist
of only hydrogen and carbon, in which case the
response factor will be about the same for all.
3) When the relative percentages of the compounds
are known, in which case proper calibration stan-
dards can be prepared.
4) When a consistent mixture of compounds is pres-
ent before and after an emission control device, and
only the relative efficiency is to be determined.
5) When the FIA can be calibrated against mass
standards of the compounds emitted.This method will not distinguish between methane and non-
methane hydrocarbons. If this is required, Method 25 should
be used or methane should be determined using Method 18
and subtracted.C019_003_r03.indd 1116C019_003_r03.indd 1116 11/18/2005 11:07:17 AM11/18/2005 11:07:17 AM