Environmental Engineering Reference
In-Depth Information
The above analysis indicates that the ECD is potentially very sensitive but is
not a universal one. It is 10-100 times more sensitive than the FID, and
approximately 1 million times more sensitive than the TCD. Many chlorinated
pesticides such as DDT are very sensitive with the ECD detector. The invention of
the ECD in 1961 by Lovelock has in fact revolutionized our understanding toward
the global fate and transport of this once widely used pesticide in the world. The
ECD is also used to detect purgeable hydrocarbons, phthalate esters, nitro aromatics,
halo ethers, and chlorinated hydrocarbons as well as halogenated gases, such as
PCE, TCE, DCE, and so forth (refer to Fig. 2.1). The response to chlorinated
hydrocarbons increases by about 10 with each additional Cl atom. But the ECD is
insensitive toward functional groups such as amines, alcohols, and hydrocarbons.
Linearity varies with conditions and analytes, and quite often the ECD has a limited
dynamic range because the response is nonlinear unless the potential across the
detector is pulsed.
Comparison of Major GC Detectors
The above-described GC detectors are summarized in Table 10.5. Several other GC
detectors with environmental uses are also included in this table for comparison
purposes. These additional GC detectors are briefly described below:
Photo Ionization Detectors (PID): PID is similar to FID, as both result in the
ionization of the analyte. However, PID uses UV light as the energy source to
ionize. It does not require support gases, hence it is ideal for portable
instruments. For detection of purgeable aromatics, PID is about 35 times
more sensitive than FID. It is also non-destructive, so it can be used in series
with other detectors.
Nitrogen-phosphorous Detector (NPD): Also called thermion detector
(TID) or nitrogen flame-ionization detector (N-FID). NPD is similar
to FID but uses a ceramic bead containing alkaline metal that can emit
positive ions when heated in a gas stream containing certain analytes. It
is sensitive to N-and P-containing compounds such as organophosphor-
ous pesticides or organonitrogen compounds.
Flame Photometric Detector (FPD): The GC effluent is mixed with H 2 O 2
that gives a hot flame. The chemiluminescene emitted from the combus-
tion of S- or P-compounds is then measured at 394 nm (S) or 526 nm (P).
FPD is sensitive to S- and P-containing compounds. It is similar to FID
but more sensitive.
Hall Detector or Electrolytic Conductivity (ELCD): S-, N-, and halogen
(X) compounds are converted to ions under a nickel catalyst heated to
850-1000 C. The conductivity of the dissolved ion in liquid is measured
for quantitation. It has been adopted by the U.S. EPA for the detec-
tion of halogenated compounds, but generally with limited popular
acceptance.
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