Optimizing the Analysis of Volatile Organic Compounds

26
www.restekcorp.com
FID Maintenance and Troubleshooting: Contamination and a clogged jet are common
problems associated with using an FID in analyses of volatile compounds, such as gasoline
range organics (GRO) analyses that involve samples containing diesel fuel or oils. Flameouts
at the beginning of a VOCs analysis usually are the combined result of incorrect gas
flows and excessive water from the purge trap. When performing maintenance on an FID
always check the gas flows before calibrating the instrument. Water management is discussed
on page 13.
ELCD (Hall ® detector) Operation
In typical applications, an electrolytic conductivity detector, ELCD (Figure 21), is a chemical
detector that catalytically reduces halogenated materials to haloacids, HCl and HBr, by
mixing them with 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):
H2 + CH3(CH2)nCH2R (heat and catalyst) –> CH3(CH2)nCH3 + RH
where R=halogen (Br or Cl)
Example:
H2 + CH3Cl –> CH4+HCl
The haloacid molecules flow into the electrolytic conductivity cell via a PTFE transfer line,
and are dissolved in a stream of n-propanol. The conductivity of the alcohol is monitored
because the concentration of hydrogen halide is directly proportional to the current. The signals
thus produced characteristically have tailing peaks. Although the ELCD is most commonly
used for halogenated compounds and, in the halogen mode, 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.
TFE Transfer Line
Reactor
Reaction Tube
Reaction Gas
Vent
Solvent
Exit
Silicone
“O” Ring
PTFE
Seal
Signal Out
Cell Excitation
Voltage
Solvent In
Conductivity
Cell
Outer
Electrode
Inner
Electrode
Tube Nut
Gas Effluent from Reactor
Outer Electrode
Cell Body
Insulator
Inner Electrode
Cell Cap
Wave Spring
Courtesy of Thermo Finnigan