The
Flame Ionization Analyzer:
The flame ionization
method of hydrocarbon analysis relies on a highly sensitive, but non-selective
detector (FID) which decomposes the gas sample in a hydrogen flame.
The gas sample is destroyed in the process so no further analysis
of the sample is possible. Variations on this type of detector are
commonly found in gas chromatographs, on-line hydrocarbon analyzers
and in portable field survey instruments. The flame ionization detector
alone will provide only total hydrocarbon data however, the detector
may be readily enhanced to provide some degree of selectivity in on-line
instruments. In GC applications, column seperation is used for speciation.
Here
is how the Flame Ionizaton Detector (FID) works:
This is a simple
concept and this example uses a simple case. We will follow one methane
(CH4) molecule through the process of ionization in a hydrogen
diffusion flame and show how it may then be detected in the hydrocarbon
analyzer. A diffusion flame is one in which the oxygen necessary to
support combustion is supplied to the outside of the fuel supply.
This results in the necessary flame geometry for stripping hydrogen
atoms from the hydrocarbon molecule and the formation of carbon radicals.
During the course
of normal combustion, a small, stable proportion of CH4
molecules will temporarily ionize into C+ (Carbon, or carbon-containing
cation, e.g. CHO+) and e- components. The flame provides
the energy for this ionization. This is generally speaking, a short-lived
condition and the charged components quickly recombine into the products
of combustion as they loose energy, moving upward, out of the flame.
However:
If this happens
in an electrostatic field, the oppositely charged components can be
driven apart, toward the oppositely charged field generators (in this
case, vertical plates, one held at a high negative potential and the
other at approximately ground). Upon reaching the oppositely-charged
plate, the charged component (C+ or e-) is electrically
completed. The Carbon molecule will readily form CO2 with the Oxygen
from the surrounding air. Note: the remaining Hydrogen is not a player
in this part of the reaction, but readily forms water vapor
Looking more
closely at the diagram below, the negative supply is an unremarkable
high voltage type which supplies the Carbon cation with electrons,
but the "ground" is the input of a sensitive current (I)
amplifier which "counts" the electrons (e-) passing
through it on their way to neutralize charge. Indirectly then, for
Carbon atoms in an H-C molecule, this scheme acts as a "Carbon
Counter". Some variation in response occurs due to species molecule
arrangement. This difference (from the "carbon counter"
model) is somewhat unique for different species. A " Response
Factor" is associated with different hydrocarbon species to correct
their measured response with a standard calibration gas.
This is a general
explanation applicable to all FIDs found in hydrocarbon
analyzers. Different manufacturers use slightly differing schemes
and geometries. Some have a single detecting plate, or horizontal
ring, some have a grounded flame tip and count Carbon "directly".
The detector biasing voltage may vary (between designs) from about
-90 to -400 VDC. The FID is usually heated to prevent the formation
of water vapor condensation from the FID flame, however a heated FID
will have a detector and sample handling components operating at up
to 200 degrees C in order to avoid both water vapor and long chain
hydrocarbon condensation in the sample gas.
Normal Combustion:
i.e. burn methane in air and get carbon dioxide and water vapor...
CH4
+ O2 --------> CO2 + H2O
or:
CH4
+ 2O2 = CO2 + 2H2O
Flame
Ionization:
during combustion, a uniform proportion (about 0.0002%) of the molecules
in this reaction do this instead:
(simplified for clarity)
CH4
+ O2 -------->C++ H2O + e-
--------> CO2 + H2O
or:
CH4
+ 3O2 = C++ O2 + 2H2O
+ e- = CO2 + 2H2O
These oppositely-charged,
intermediate products can then be detected by the FID:
