The FID/DELCD is one
of the most useful detector combinations because it allows the operator
to reliably identify hydrocarbon peaks detected by the FID
as halogenated or not.
While less sensitive
than the ECD detector, the DELCD is much more
selective, eliminating interferences which would complicate an ECD analysis
(such as ambient oxygen). Sample preparation which might be required for
ECD work is not required for the DELCD because of its total selectivity
to chlorine and bromine, and because the FID pre-combusts any contaminants.
In the high sensitivity mode (hydrogen off, using dry cylinder air), the
DELCD can detect down to the low picogram range. In this mode, the DELCD
is about 100 times more sensitive than when used with the FID in low sensitivity
The DELCD measures
the ClO2 present in the FID exhaust gas.
Because the FID combusts the sample upstream of the DELCD, all hydrocarbons
are comverted to CO2 prior to the DELCD,
thereby preventing large hydrocarbon peaks from contaminating the DELCD.
Since the DELCD operates at close to 1000°C, it can tolerate
the water-saturated FID effluent and measure the chlorine and bromine
content simultaneously with the FID hydrocarbon content measurement. This
is especially useful for measuring chlorinated VOCs under a solvent peak,
or measuring PCB peaks obscured under large amounts of diesel fuel. This
detector combination is often used with our Thermal
Desorber and Purge & Trap, which concentrate
the sample to achieve lower detection limits (see
our preconfigured PCB GC system on page 19).
The FID/DELCD is supplied
with dual amplifiers.