26
FIDMaintenance andTroubleshooting:
Contamination and a clogged jet are common
problems associatedwith using an FID in analyses of volatile compounds, such as gasoline
range organics (GRO) analyses that involve samples containing diesel fuel or oils. Flame-
outs at the beginning of aVOCs analysis usually are the combined result of incorrect gas
flows and excessivewater from the purge trap.When performingmaintenance on an FID
always check the gas flows before calibrating the instrument.Watermanagement is dis-
cussed on page 13.
ELCD (Hall
®
detector) Operation
In typical applications, an electrolytic conductivity detector, ELCD (Figure 21), is a chemi-
cal detector that catalytically reduces halogenatedmaterials to haloacids, HCl andHBr, by
mixing themwith high-temperature hydrogen in a heated nickel reaction tube. In other
words, this detector pyrolyzes these analytes in the presence of a catalyst and a reaction gas
(hydrogen):
H
2
+CH
3
(CH
2
)
n
CH
2
R (heat and catalyst) –>CH
3
(CH
2
)
n
CH
3
+RH
whereR=halogen (Br or Cl)
Example:
H
2
+CH
3
Cl –>CH
4
+HCl
The haloacidmolecules flow into the electrolytic conductivity cell via aTeflon
®
transfer line,
and are dissolved in a stream of n-propanol. The conductivity of the alcohol ismonitored
because the concentration of hydrogen halide is directly proportional to the current. The sig-
nals thus produced characteristically have tailing peaks.Although the ELCD ismost com-
monly used for halogenated compounds and, in the halogenmode, it is selective only for
these species (Figure 21), it can be configured to detect sulfur, nitrogen, and nitrosamine
compounds. Figures 22-25 and Figure 27 show various important parts of the ELCD system.
Figure 21.
ELCD configured for detecting halogens.
Solvent In
TFETransfer Line
Vent
ReactionGas
ReactionTube
Reactor
Solvent
Exit
Inner
Electrode
Outer
Electrode
Conductivity
Cell
Silicone
“O”Ring
Teflon
Seal
Cell Excitation
Voltage
Signal Out
Wave Spring
Cell Cap
Inner Electrode
Insulator
TubeNut
Gas Effluent fromReactor
Cell Body
Outer Electrode
Courtesy of ThermoFinnigan
