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# Method 25A - Gaseous Organic Concentration (Flame Ionization)

## What is the correct calculation method to convert ppm on a weight basis from a Method 25A test from 'as propane' to 'as methane' or from propane to 'as carbon'? Some documents state to multiply by 3 to go from propane to methane since the FID counts carbons. If so, then to convert the same VOC to 'as carbon' the numerical value would be the same expressed 'as methane,' and that does not seem legitimate since the molecular weights of carbon and methane are different.
For gaseous samples, such as those analyzed by Method 25A, ppm is normally a volume/volume fraction, not
a weight/weight fraction. I am not aware of anyone expressing Method 25A results as a weight/weight fraction.
For ppm on a volume basis, molecular weight is not an issue because a mole of any gas has a volume of 22.4 liters
at standard temperature and pressure (STP) regardless of the molecular weight. Because the response of an FIA is
approximately proportional to the carbon atoms in a compound, the FIA response to a mole of propane is approximately
three times its response to a mole of methane (there are three times as many atoms of carbon in a mole of propane as
there are in a mole of methane). The volume of a gas is directly proportional to the number of moles, so if the
response of one mole of propane is equivalent to three moles of methane, then the response of a liter of propane is
proportional to three liters of methane, and because it's a volume/volume fraction, the response of a ppm of propane
is equivalent to three ppm of methane. Therefore, you can convert ppm of propane measured by an FIA to equivalent
ppm of methane by multiplying by three. Similarly, as you noted, ppm methane would be equal to ppm carbon.
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